I don't have tattoos on my arms but have about half my back covered and never have any problems with them. I don't think you'll have any problems with it stretching out and looking strange unless you plan to gain serious size. If your really concerned ask the guy doing your tat. they can always touch it up. when you get the tat ask him to use the magnum.
I got my tat when i was 17....so a couple years later I have gotten alot bigger. I just went to get it filled. I guess it matters what kind of tat u got. Some shit you cant fill