There are good nematodes, microscopic roundworms, and there are bad nematodes and although the good ones outnumber to baddies I have always wondered how the people that sell them keep them separate. As
I recall it takes a very large number of Sod Webworms to produce visible damage in lawns.
Grubs, if the population is 10 or more per square foot, could cause thin spots in lawns and may need some control, but that is mostly in soils that lack adequate levels of organic matter. If grubs are the problem those you have now will be too large for most all organic controls and will soon be pupating to become the adult beetles that will be mating and laying eggs for next years crop.
What is the pH of the soil?
How much organic matter is in that soil?
Are the major nutrients, P, K, Ca, Mg. in balance?
Those are where I would spend my time on.

The 90/10 mix that you referred to was most likely 90% tall fescue and 10% kentucky bluegrass. If you do have any sod webworm damage it would most likely be confined to the kentucky bluegrass. Reseeding with tall fescue that is endophyte enhanced (such as Primetime)will increase the tolerance of any sod webworms. I would not waste my money on the biological grub control, if you do see some damage in the fall just keep the sod wet for a couple weeks and it will be fine

Many people become spooked by the white flies or moth looking critters in the lawn. By and large those are harmless. Web worms form a web in the grass for protection from birds. If you don't see webs, then you have some other issue. If you do have webs, then there is a biological control called BT Worm Killer. It contains bacillus thurengiensis. BT is a disease in caterpillars. In my experience it kills them on contact but that is supposedly not the mechanism. One small bottle is a life time supply and it lasts for 20 years on the shelf. If you mix a little molasses in with the spray for the BT, it seems to dramatically improve the longevity of the spray.

Grubs that live in the soil can be killed with the Guardian nematodes. This is a good time of year to spray for fleas and ticks. If you are concerned about lawn grubs, wait until May or June. If you see a lot of beetles swarming your porch lights, that is the time to spray. Those beetles (either June bugs or Japanese beetle) are the buggers that get to your grass from underneath.

90/10 is the mix of fescue to Kentucky bluegrass. That is measured by weight. Due to the weight differences between the two seeds, it provides about a 50/50 coverage.

I have not heard of keeping the lawn wet to kill grubs. Interesting approach if it works. One thing that will happen by keeping the lawn wet is all the weed seed in the soil will sprout.

Large populations of white moths only at a certain time of year are harmless? Everything I've read indicates that these are the adult forms of webworms. The small (but numerous) patches of lawn that died off in those areas seem to back up that hypothesis.

If I had a problem last year, isn't it a good idea to take a preventative stance this year?

My understanding is that in the first year of a new lawn/new house it's a good idea to treat for grubs so that I can get a handle on the situation quickly.

ALSO, I thought that waiting to treat once moths/beetles are already in the adult stage is much too late?

The manufacturers and purveyors of the many poisons sold to control many things will tell us that we need to purchase and apply their products at the first sign of an insect, but those of us involved in organic growing know this is not true.
The first thing to do if there is an indication of a problem is to take a good, in depth look at the soil to see how close it is to being a good healthy soil which will help what is growing there fight off insect pests or plant diseases without pouring poisons on the plants or soil. Sometimes we may need to take some action, but most often none is necessary since, for insect pests, predators will move in and control those pests providing we do not make the environment into something that will kill off the predators as well.
What you need to decide is if the damage done, mostly cosmetic, is worse then the treatment needed to change that.

Kimmsr- while I agree with you, I'm guessing you disregarded my original post where I mentioned that the "product" I was thinking of applying was Beneficial Nematodes....which to my understanding is organic?

Nope. As I wrote earlier spending money on nematodes may be a waste of yours. Sod webworms are not as big a problem as the purveyors of "cures" make them sound.

Nope to what? It's not a chemical...it is a living organism and therefore organic. Anyways, my point is this- if the lawn is new and I agree with you, needs the soil improved, but at the same time is battling against a large grub population, isn't it important to treat the grubs as well as improve the soil structure?

And my original question was whether last year's massive problem would likely recur this year, and how I should go about dealing with it. I don't really see how improving my soil at the cost of letting large portions of my lawn get decimated by pests will really help the overll picture, which is ultimately to have a healthy and yes, good looking lawn.