How detailed should we do math calculations?

Is it ok to use a pen and write down multiplication or division (as we used to learn in school) and do detailed but correct one or should we calculate in our head and write estimated number?

example:

2.7 x 3,600 =

7.5% of 85=

I am more comfortable to write down, spend 10 sec but to get a detailed answer, but not sure whether it is ok on case interview and interviewers need people who can calculate in their heads...