Using 57-559 (26x2.25) tire instead of 54-559 (26x2.1) on a mountain bike? I have a ZAC 79 front rim with a Deore HB-M530. I just found out I've got a flat tire (after thinking last week "Hmmm, probably I should get a new tire soon..." :D). I'm going to buy the Schwalbe Smart Sam Plus HS 476. However one thing struck me - I actually have one Smart Sam Plus on my rear wheel and it's of size 54-559 (26x2.1) while actually one the front I have one 54-559 (26x1.95) no-name tire that I've taken from a previous bicycle of mine. :D Both fit fine so I was wondering what the actual margin is when it comes to picking tires and whether I should by a 26x2.25 one for my rim. Speed gain (thinner tire better than fatter tire) is not such an issue here (I hope :D). I'm using my mountain bike on paved roads mostly though sometimes I also ride through woods (nothing too rocky) etc. and that's why I picked these tires.
You've got some margin for tire width—at least 10 mm (although I don't know whether your current tires are at the narrow end or wide end of that range). It's also worth pointing out that the width printed on the tire can vary by a few mm from its actual width.