my advice here: do NOT do such calculations in your head during an interview! You will not get ANY bonus points for this! The only two things that count are
(1) You are consistently correct in your math
(2) You can be easily followed
For this, do WRITTEN calculations. When properly done, it barely takes more time. But it is a much clearer way of communicating - and this is what is much much more important than speed! Doing math in your head is absolutely not client friendly: the interviewer will assume that this is how you will also act with clients. If the client can not follow you (because you are doing math in your head and are not being explicit with it, it is your fault!
That being said - it is certainly not harmful to practice such multiplications tables. But in the interview, please do the math properly in WRITTEN form!