Anyone (titled players included) really only need to know openings they play or might encounter. A chess master who only plays 1. d4 doesn't need to know deep theory into the Sicilian Poisoned Pawn Variation.
A strong chess player usually knows their openings really well and just competence in what they don't play or are able to "figure it out" in the spot for positions completely new to them.
By IM and GM level, all of them have no doubt already created opening repertoires and taken things in the opening far more serious than the average chess player. Therefore, their "competence" is a lot higher in what they don't know, but still the emphasis is on what they play themselves.
Learning openings you don't play can teach you a lot though; this is one reason GMs know a little about all openings. Sometimes, I find thematic patterns in openings I don't usually play and then a similar motif comes up in my own game! Titled players have such a huge "mental database" of positions and patterns they have seen and no doubt this helps them navigate every stage of chess (including openings), but it isn't the specific need to require openings that gets them there. You are correct, there is little need to study 1. h3 deeply if they don't want to do so, but knowing 1. h3 might teach you useful things and maybe you'll trick someone into it via transposition 
So, I am not the highest rated player, but I have been wondering about chess--and the advice I have been given about--for a while recently. One question I have is, and... I realise this is a question that comes up over and over and over and has no definitive answer, but, what actually are the best openings? I don't mean like 'What is the best response to 1.e4?', 'Oh, Black can reasonably choose, 1...e5, 1...c5, 1...e6, 1...e6', etc. Or, 'What is the best response to 1.d4?', 'Well, Black's most usual replies are 1...d5 or 1.Nf6'. No. What I am wondering is what are the chess opening which are so essential that they have to be understood--[prerequisite tactical and positional not withstanding]--by masters of the game.
There is of course going to be some great variance in what each master understands; between individuals; and, between more general levels of ability. For instance, super-GM's have a far wider and deeper grasp of pretty much all positions, than regular GM's do (without any disrespect meant here). And that it is the same point from GM to IM and so on and so forth. And that each individual will have certain favourite openings and pet-lines.
What, however, does a master need to know to some extent? Everything? Do masters know theory in openings like the 'Polish', or 'Grob', or do principles and competence just take precedence? Is it 1.e4, 1.d4, 1.c4, 1.Nf3, and all main and (the best) side-lines? and the same backwards? There must be some point at which everyone says, 'let's not bother studying that'. No point working one 1.h3, y'know? At some point you need to sleep.