Can using SYS 64738 hurt my 64 if I use it too much?
Not at all. In fact, it's much better for your computer than the alternative flipping the power switch off and on.
Typing SYS 64738 and pressing RETURN on the Commodore 64 triggers what is called a cold start reset. This has virtually the same effect as shutting off the power and turning it on again. Memory locations are reinitialized, and any data in Random Access Memory is erased. Control is then returned to BASIC. Note that this is quite different than pressing RUN/STOP-RESTORE to trigger a warm start reset, which does not erase data in RAM.
Using SYS 64738 to reset the computer saves wear on the power switch and has no harmful side effects whatsoever. You can achieve the same results on a VIC-20 (with or without memory expansion) by entering SYS 64802, and on an Atari by typing POKE 580,1 and pressing the SYSTEM RESET button.