You know your army is in trouble when...

[ComradeP]

It's mostly an issue at more than 1 quality level difference between the assaulting units and more or less trivial with the difference from A to B or B to C due to it being only 10%.

1/3 additional assault forces at a cost of a 20% reduction in assault value is fine.

With the Soviets, it's a bit tricky as corps/army troops might be D while Guards are C, so mixing D quality engineer/sapper units with C quality Guards may or may not be worth it. Generally speaking, throwing more bodies at the Germans tends to be the right strategy compared to trying to launch only same quality level assaults.

Due to results being rather variable, it can also be better to assault with (for example) 3x2 units than with 2x3, depending on the strength of the defender.