I would use washers especially on the drive side which if you're building a low spoke count wheel the drive side is the only side that matters only if the rim doesn't have ferrules or sockets. You should also grease both sides of the washer so that the nipple turns easier which in turn puts less stress on the washer as well as the nipple and spoke. Some argue that some wheels have thicker walls at the hole instead of ferrules or sockets, this is suppose to eliminate the need for washers, but again on the drive side I would with those thicker wall models.
Todays wheels have more dish due to adding of additional gears, this dishing causes less stress on the non drive side but it also means that the spokes will loosen over time, so to compensate that loosening effect you have to apply more tension on the drive side without compromising the rim which the washer allows you to do, this in turn will allow you to increase tension on the non drive side to prevent the loosening.
If you decide to use washers only use brass, the softer nature of brass allows the spoke to set better.
I happen to subscribe to overengineering, therefore I would buy wheels with eyelets unless it's a deep profile. Eyelets like washers aren't really necessary except for the drive side and since they add a tad more weight people dismiss them, but for long term reliability, and especially for a heavy rider or a heavy load I would get a rim with eyelets.
There is a wheel builder on another forum that discussed this to some length you might want to read what he says, he goes by the handle Mike T and is very well known for building high quality wheels; see:
http://forums.roadbikereview.com/wheels-tires/lets-talk-about-nipple-washers-315229.html
