Is that just a matter of preference, garden size or type of soil?
Or even no tilling! I have a mantis tiller my MIL passed on to me (it needs a tune-up, but if anyone can use it, I'd be happy to pass it on). I don't want to till my soil, although it is typical VA heavy clay, because it DOES have alot of worms. My approach so far has been lasagna/sheet composting-I started a compost pile a year ago-grass clippings, shredded leaves, kitchen trimmings, bit of ash from the fireplace. When I want to start a new bed (or work on an existing one), I lay brown paper or newspaper, then compost, topsoil, humus, mulch, whatever....I have alot of large trees (& roots), & I'd rather just dump stuff on top then try to till the soil (this clay is wicked!)-some plants make it, some don't...
Unless it's an elevated bed I always use the tiller, even when I make an elevated bed I'll sometimes till the soil underneath. The only areas that I don't till are the beds around trees where I could damage the roots. It doesn't seem to bother the worms and I always have plenty in the soil, especially after it's been tilled. The nasty red clay is everywhere. I've never tried the lasagna beds and will probably try making a few this year without tilling the soil first. It's exhausting making a bed in an area of the yard that hasn't been touched in 40 years even with the tiller.
Rototilling more than oncee a year for existing beds, or two-three passes for new beds, can create a compressed hardpan surface under your bed where the blades scrape bottom. That said, I have a monster tiller I use to loosen the top six inches of clay for new beds -- I then pile on six inches of good stuff, run one pass to mix that in, and a second pass to mix in six more inches. Most of my beds never get tilled again once the perennials and woodies get planted; just top-dressed with manure and leaf-mulch each spring and fall, which the worms mix in. Over the years that has turned the clay into more than a foot of good loose black crumbly loam. The clay adds valuable minerals, plus keeps the beds from drying out in drought years; the beds I have made by just going UP with manure/leaves/sand dry out pretty fast -- good for some things like salvias and crinums, but not for everything. My veggie bed gets a green manure planted in the fall, and that plus another six inches of good stuff tilled in -- one pass-- in the spring.
I also have a mantis-type tiller I just love. Best thing for a five-footer like me to use to dig holes for trees, or to loosen the footer for new walls. It's like a frantic Jack Russell Terrier on a good leash.
I think I'll just rent one this year and till the whole thing, put Leafgro on there and till again ebfore planting all my veggies.
Here's one vehement vote for _never_ tilling. I've been gardening for 40 years and have always had excellent gardens by loosening the top few inches of soil with a shovel when necessary, and layering organic matter/compost right on top. Even for preparing new beds over existing grass, you can do lasagne gardening (google that phrase and check out the book(s) on the subject) without removing the sod or tilling.