I don't use a mounting block. I never got around to making one, refuse to pay $60+ for something I could make myself, my horses are practically midgets (14.1 and 14.3) and at this point in life I'm pretty light about popping aboard from the ground. Not textbook-ideal, granted, but I've not had a problem. If there's a handy stump or low spot, I do take advantage of it. I keep my girth quite firm; I can get my hand between it and the horse, but the saddle certainly isn't going anywhere. After I mount, I generally step into the off-side stirrup to rebalance a bit, and everything's good and straight.
Most English riding instructional books, and a lot of dedicated-English riding instructors, very highly recommend using a mounting block or having someone hold the stirrup on the off side to prevent the saddle from torquing or from pulling the horse's spine. I've always read/heard/been told that the difference is that the western saddle has more surface area and a sturdier tree that makes mounting from the ground less potentially injurious to tack or horse. I've seen English equitation classes in which the riders were asked to dismount and remount where the ring steward held the off stirrup for each rider, and a block was provided for the truly tiny riders or ginormous horses.
Some horses, usually those that are either carrying some extra weight or have naturally low, flat, or mutton-withers, just always have a little saddle slippage during mounting, whether the girth is downright sloppy-loose or squeezing them in half. A little slip (i.e., pulling the saddle off-center, not twisting halfway down the horse's barrel) is to be expected, IME. Just step in the off stirrup to realign and go on your merry.