For example, with an SAT score of 2100, is it better to have all 700s or a 800 M, 600 CR, 700 W?
All 700s
The writing score is not considered as heavily by some colleges. So you are best off with higher CR and M scores.
There are many colleges that do not use the writing section to determine admission, and thus in your example your best score would be 800 math, 700 CR, and 600 writing. That could also be true for many others at which the writing score is not considered as important as math and CR.
What about a scenario in which Math and CR are the same? For example, 700 and 700 vs. 800 and 600?
I would rather try to do the best I can on each section, than worry about whether “sacrificial” 700s are better than 800/600.
For the record, my first SAT was really imbalanced (800 Math, 590 reading).
My SAT is 650 CR, 740 Math, 690 Writing, 2080 total. I’m not sure if that is considered balanced or not.
Whoops I meant 800 Math, 570 reading (after remembering what I got).
I don’t know how to define “balanced” or “imbalanced,” perhaps if two scores differ by at least X or the standard deviation of your scores is at least Y, idk. I don’t think it really matters much, unless applying to top schools that have high median scores. Everyone has strengths and weaknesses.