Interesting question. 🙂
I like the look of natural, cosmetic-less feet (save for the general health and care for the skin and whatnot, so would prefer no polish on the feet that I'm tickling/worshipping/etc. Though that's only a preference - I'd not mind it either way.
For 'lers who are tickling me, I'd think the polish would benefit any roleplayed scene or something - that is, dark, intimidating nailpolish for the ruthlessly sadistic Mistress, a striking red for when said 'ler wants to be particularly sultry and teasy...err, bright or multi-color goes well with pigtails? :lol But as far as straight-up, just for fun sort of play, again, nothing can beat the healthy look of natural fingernails. 🙂