Hi Raj, 

          Look UPSC services contains total of 7 papers in stage two(mains exam) .  Out of 7 subjects, 5  subjects (essay, history & geography (gs 1),polity (gs 2),economics environment & disaster management (gs 3),ethics (gs 4)) are already given by the UPSC, which common for all. In rest 2 subjects, you can choose it according to your interest or that you want to take.

So if you do bsc in maths, if you choose maths in optional, then it will be beneficial for you. Otherwise you have Quant, logical reasoning etc in the stage 1,in that case also you will be helpful.

Look most of cases, in the optional subjects, candidates got good marks to maintain their overall results. 

So, hope you understood.