first of all, let me state that one result of proper case interview preparation will be that this question will become completely irrelevant to you. ALL calculations in Consulting case interviews are utterly simple! You just need to internalize these important points and act accordingly:
1. You NEVER do calculations in your head! Interviewers are not at all interested in your ability to do calculus in your head! The only thing that counts is that you can be trusted to consistently come up with the correct result.
2. Don’t worry about speed! Again, the interviewer wants to see you come to the correct result in a very secure way - NOT in a speedy way
3. As a consequence of the above, concentrate on practicing written calculations of all sorts. All 4 operators (+, -, x, /). Thereby you will quickly get to the level where it becomes completely irrelevant whether you solve for 43% of 90 or out of 100.
If you still want to get a rule of thumb on rounding, I think it is permissible if well below 10%. In your example 100 is clearly more than 10% above 90, so it seems like a stretch to me.