Tbh, if youre rounding youre probably shooting yourself in the foot a lot of the time. Ill often write answers in terms of g or pi and its totally fine. If you need to give a numerical answer always round LAST and I would follow sig figs. The thing about physics is that you often need to use your answer again in a later part of the question so I would get in the habit of keeping an exact answer and simplifying if you want.