I've never noticed much break in taste with rayon and I'd be hesitant to try and boil or clean it either, but to be honest I can't much compare it to cotton which I BARELY used because I just couldn't wick it right.
I will say you are getting good advice, and I generally saturate my rayon and wick with juice completely and then do some pulsing. I mainly do it to check my build and wick and how it is looking, etc., not so much for taste, although I will say, perhaps that is why I find little taste issues with rayon to begin with.
Improper wicking with rayon (too much and not thinning the tails quite a bit) could probably cause flavor issues in the sense that you have a ton or rayon in there, not really engaging in capillary action (wicking the juice from the tails to the coil efficiently) and you may wind up with "Yuck." I have a few times, even when I'm pretty mindful that with rayon in many ways (other than the coil) less can be more etc. It's just more efficient at wicking when there is less of it choking up a tank.
It is also possible to over wick even in the coil (I did it early on, even though rayon plays to my strengths, honestly, the desire to overstuff the coil.) After a few early "wick fail need changes" I went to the rayon thread (my wicks were breaking in two, inside the coil) and learned to completely wick less. Literally EVERY time I wick with rayon nowadays, I go, "That can't possibly be enough," LOL and it is always, entirely enough, and those are my wicks that are leak free AND last the longest, crazy amount of long, especially in larger atomizers, where heat is more diffused.
But, I love rayon dearly. I've only ever used the "Sally's Graham 100% rayon Cellucotton" and it has served me well. . I have wicks going on weeks admittedly I DO have a lot of setups but rayon is great, once you figure out care and handling. I am SO glad someone pointed me to the rayon thread, 2 questions out of me, 3 answer, and then again when my wicks were breaking in two, well I was SET for every RTA I have.
I do think many things have a break in period for sure, but there are things you can do to lessen it. Etc.
Anna