- I think it's kinda rude to shuffle your opponent's deck at FNM. It's an FNM, for goodness sake -- just cut. If you really think that people are cheating at your FNM, then I would find a different place to play, because those people sound horrible.
I shuffle at FNM, but never because I think my opponents are cheating. If I thought that, I'd call a judge...well, one of the other judges...and investigate. I certainly wouldn't shuffle and thereby destroy any possible evidence of their cheating.
Firstly, I do it because shuffling is a habit I don't want to lose--if I stop shuffling at FNM, I'm going to forget to do it elsewhere when the stakes are higher and it's far more likely to matter.
Secondly, I do it because sometimes my opponents mana weave or pile "shuffle". It's FNM, it happens, and the deck should really be shuffled properly afterwards. Doing it myself means I don't have to sit there and argue with my opponent for ten minutes about how pile counting isn't really shuffling and no, mana weaving is
not okay "because it's not stacking the deck, it just stops mana flood" in order to get a properly shuffled deck across the table. I can educate later--right now, I want to play!
Thirdly, I do it because if I
don't always shuffle, then in the aforementioned cases of weaving/piling where I
should shuffle, doing so both looks and feels like I'm accusing my opponent of something, whereas if I always shuffle regardless, it's just the way I do things, much the same way I'll always go for evens/odds dice rolls to decide who goes first rather than high-number.