Finnish does have allophones, but there's no regularity whatsoever from what I can tell; linguists and linguistic sites will all tell you that Finnish has, as allophones of [h] at least [ħ] and [x], but some people never pronounce the "h" any differently in any word, whereas others will even have [χ] in some words. The "r" is also supposed to be always trilled, [r], but I couldn't count the times I've instead said [ɾ] or even [ɹ] or heard others do that. With vowels, it's even less regular, and it varies by dialect more than consonants; while "hän menee" (he/she goes) is /hæn meneː/ standardly, it could also be /xən məneː/, /hæn mennøː/, /xæn mennøː/, /xən mənnøː/... but that's not really an allophone thing, just dialectal differences. Still, saying that Finnish has one-to-one correspondence between writing and pronunciation isn't entirely accurate.
...but in the context of conlangs, everything's possible, so I don't know why you should limit yourself in any way.