Well, I have two answers. If it is mental math I use 3.1 and round up. If I am calculating something I care about I use my TI-86 which has pi to 14 digits.
Practically speaking, I don't often need to convert diameter to circumference but I do occasionally need to calculate area or volume and in those cases I have way more error in other measurements or assumptions (2 or 3 digits) so 5 digits of pi is more than enough.