Enchanter cha impact on charming, worth going from 252-255?

[tranceminus]

Hey all, so I remember reading somewhere on the forums a while back that the bottom line effect of charisma is that every 10 points gives you effectively -1 MR on your resist checks and charm breaks.

If this is true, and assuming that you aren't able to get partial points off MR (-0.3 MR for 3 points of cha for example) then does that mean that there is no effective difference between 252 cha and 255 cha? Or, for that matter, between 250 cha and 255 cha?