I've never used spoke prep, but use a q-tip to put Tri-Flow on the threads and the contact area of the rim, just to make adjustment easier. I've never had a rim lose adjustment.

However, on the occasions when I've needed to replace a rim, I'll be damned if it seems like the threads are full of fine dirt. Damned if I can figure out how that happens, but it makes me consider using something heavier than Tri-Flow to eliminate the need to clean out the threads when doing rim replacements.