I think the saying is only valid to the point that once you had high nitro content you will not go back to lower. I can attest to that! For my Saito its 25% and 15% feels sad its not fun at all ... the difference is tremendous.

For your 46AX 15% is just nice, no point going anymore, going less does not harm the engine for sure but too low and sometimes tuning is not as fun.