I always retire requests that have been filled, because it feels awkward to request something you already got (especially when you match the same person again.) I sometimes repeat others, but I also basically only do Yuletide. But I thin most people who do tons of exchanges are used to repeated signups, and most people who only do Yuletide don't care.

Do tweak it some though - there are enough people who only do Yuletide that things that are now SOP in letters in exchange culture may not be decipherable to a yuletide-only writer.