I won't claim to know who is right in this matter, but can see that the 2 of you see things differently.

She sees a winter mess as opposed to a neatly mulched area all ready for spring. (I'll bet she is an A personality, like I would 'like' to be.)
You see an big expanses of mulch, with no interest.
My beds usually don't get the cleaning and mulching I would like for them to get, and I would like them all cleaned up...like your co-worker.

I 'think' it is in the irises best interest to cut them back (I could be wrong here)as dead matter around them can promote ill health. You might search or ask at the Iris Forum. It may be a bit late to do that in your zone, but again not sure. I hope to do several of mine yet within the next week, as the temps are to be 60 for the next 7 days!

I do suggest leaving the lavender and thyme alone, as it comes back I think to a large degree on the old growth, besides having winter interest.

I imagine by now the coreopsis (and maybe some others) have reseeded like crazy, and will be everywhere next spring. Mine reseeded this fall, and is now everywhere! If it was me, I would cut it off, but that is just my 'preference.'

It is best to cut any mums that were planted this fall. If the stalks are left, they will blow and cause them to become loose in the soil with the many freezes and thaws, and they will freeze out. Best too to scoot a little extra mulch up around them too. I'd think that the taller sedums, may do similar things too if not established well.

For winter interest, could you add some accoutrements (sp)?

Bird feeder on a shepherd's hook, or even something placed on the ground? Little sign for the season? I have the sweetest concrete Santa, but know you might risk having something like that stolen. Possible some geodes, or other rocks?

Some plastic or silk poinsettia?....just kidding on that one!

If it makes her feel good, why not let her go ahead, but ask that she leave the thyme, lavender, and the iris until you can check on them.

Good luck, I hope you can come to a happy compromise.

Sue

Some perennials should be cut back, and some not. Chrysanthemums and lavendar being two which should not be cut back until spring. Chrysanthemums need the old growth to protect the crowns to preserve them for the next years production, particularly in areas where the snow cover isn't reliable (like...it all blows away. :) ).

Iris should be cut back, and the leaves removed, as well as hostas to prevent (or reduce) the number of nasties overwintering, whether it be bugs or disease. I have also found that cutting back Siberian iris helps prevent (reduce) the amount consumed by voles as the leaves no longer provide cover for the nasty eating machines.

I do leave some stuff up for winter interest, and some stuff I just don't get to all of it. Sedums and grasses I don't cut down until they start looking ratty. I don't cut down things which are tender.

I do intentionally plant things for winter interest, having somethings which either are structurally interesting or their primary period of wonder is winter--such as Winterberry (Ilex verticilata) which drops its leaves showing wonderful red berries on bare stems, Siberian dogwood (also red and yellowtwig dogwood), evergreens, and Harry Lauder's walking stick which is pretty boring in summer, but the wonderfully contorted branches are neat in the winter.

I mix all the above with perennials such as german iris, daylilies, helebores, any everything else you can think of, interspacing with early spring flowering bulbs, in order to try to give me more bang for my buck.

It takes planning, but it is well worth it. I have a friend who does so well at this that her garden only looks a bit tatty in early March, by which time most of her grasses have gone icky looking. Goodluck, and Sue's correct, it is partly a difference in gardening technique. Unfortunately, I'm like you...living next to someone who whacks everything back and even trims the tips of her prodigious collection of daylillies. Maybe in my next life I'll be able to accomplish all I want to in the fall????

I should say, I do deadhead regularly, and clear out dead plant material as it turns brown -- I'm not totally slovenly! But I don't like cutting back plants that are still green -- and despite a few nights of weather in the 20's we're back to warmer weather. It was nearly 60 yesterday!

I thought she cut back the chrysanthemums too severely -- you can't see where we planted them!

I know I should trim the irises -- I lost a nice patch of mahogany iris at home to iris borer, and I'm sure it's because I didn't keep up with grooming them!

I guess it's like most things in life -- we need a little compromise on both sides!