Remember that as observers we are deciding if referees are ready for the next level, but also making sure they are ready and prepared for that next level. As I said, at Level 4 and above you will rarely if ever see a referee with his shirt untucked, so it would be remiss of observers not to point this out. That doesn't mean that they get a lower grade as a result of it, it can be mentioned without having to go to that level.
At level 4 they will be working most games with level 3 referees, these guys will have worked hard to get to that level and will have coaches / mentors. I know from experience that they will preach the mantra first impression and appearance matters, and the majority of them would tell (not ask) an assistant to tuck his shirt in. It goes much further, they often try to make sure all officials have the same kit, sometimes provide warm up tops so that all three look uniform, all having socks either fully up or rolled down in the warm up, I've even been asked in the past to wear a certain coloured shirt and tie so that we all look the same (that is going too far in my opinion). I don't mind admitting that when I was a L3 if I asked an assistant to tuck in his shirt and he refused I'd be reporting that to the FA. It never happened, but at the end of the day it was my club marks that would get affected by him looking sloppy, not his as the clubs only mark the referees.