ehhh why don't sig figs matter in physics?! >=0 Do you mean as in the marking scheme for the external exam?
They mattered verily much for my school assessments though. But I really hate the inconsistency with which they preach sig figs.
Basically, you can only keep the lowest no. of sig figs, but the stupid thing is sometimes there's only 1 sig fig in the question (e.g. rocket blasts off at 5m/s). In that case, they sometimes go like OH NO TOO FEW SIG FIGS IN YOUR ANSWER, so just write a few more. Stupid but that's HSC physics for you.
Most importantly, in your option topic, if you do Quanta to Quarks, you must remember that sig figs do not apply for addition/subtraction (e.g. in mass defect). In that situation, you keep the least number of decimal places instead.
Also they have really funky rules for sig figs in chemistry when you take pH (since you take a log base 10). I think from memory you keep the same number of decimal places as the significant figures in the question's original number. Look them up in your textbook to be sure.