Yep, this is an urban myth. The idea behind it is only for ABx engines and goes like this. When the engine gets a bit worn the piston to liner clearance gets larger. Because the liner in this type of engine has a taper the assumption is made that less nitro gives a lower engine temp so the liner doesn't expand as much at the top so the piston then fits better to regain compression.

But the fact is that IF there's any difference in running temps between say 5% and 10% nitro then the clearances might change on the order of millionths of an inch. The thing that actually gives the final seal against compression is the oil film between the liner and piston. ABx type engines really aren't as critical as some people think.