In general, a smaller capacitor will be able to get closer to the pin and therefore do a better job decoupling. I think it would make sense to just use as few 0402s as possible: the probability of making a mistake soldering is just going to be increased for each additional capacitor. I don't think we have to think about it like reducing trace width or such (therefore raising the cost of the whole board). When these boards are eventually loaded by a professional shop, they won't even blink when we ask them to load 0402s.
Seconded that pro shops won't blink at 0402s. If there's going to be a hand-soldered prototype, avoid them unless you absolutely need them. Later on, switch to 0402s to reduce board space and cost (assuming large quantities of boards, of course).
I rework 0402s all the time. My suggestion: when laying out a prototype that has to include 0402s, like in the bypass capacitor example above where using a 0805 might cause a difference in behavior, make sure to leave room around the 0402 so you have room to get a soldering iron in there. Just because it's a small component doesn't mean that the board needs to be packed densely, and densely packed 0402s make it tough to only remove the desired resistor without accidentally removing nearby resistor(s). Those little suckers come on in like two seconds if your soldering iron strays. A quality pair of tweezers and a magnifying glass for inspection are also must-haves.
FWIW, I use one of these cheapo $90 USB "microscopes", which is a handy gadget for documenting issues:
http://www.amazon.com/ViTiny-Handheld-Microscope-measurement-functions/dp/tags-on-product/B003NGIBQI
and a pic of a 0402 cap that is accessible enough, with enough space to the left & right for soldering iron:
