It depends on several things. If Android, then you'll have to do flash system.img too. Yes, build and flashing system.img take a longer time than other images. If you just want to add TAs, then as suggested earlier, you can just
adb push the new files. Another way to tell is to check your build and see which images have been updated, i.e those updated have to be re-flashed. If many things have changed and you're not sure, then safer to flash everything again.