Both of my cars use flat metal washers (and neither one is something the local shops stock). Recommendation (manual) is to replace every time. I've never replaced either one (1991 and 1995 vehicles). Never a single drop of oil from either. If I did leak a few drops I wouldn't get too excited anyways since on the '95 the turbo oil return line has always dripped a little (replaced and resealed twice now to no effect, appears to just be a poor design).
However, my motorcycle (1994) has a crush washer and those are stocked along with the oil & filter, so it's easy to pick one up each time. Plus, even a drop of oil leaking in front of my rear tire is a crash risk I don't want to take. That said, I have reused a crush washer on it before. It's very hard to get the right oil level in the bike, air pockets for easily and it's easy to overfill, so I've had to take the filler plug out, drain a little, then stuff it back in (reusing the crush washer). No leaks as a result of that yet, so perhaps a crush washer can, in practice, actually be used a second or third time. Either way, I keep a sharp eye out for any leaks on that vehicle since it could be so disastrous.