Yeah, completely dependent on the situation - particularly the players, but also things like general table atmosphere (casual or intense), and frequency of play. But in general speaking of an 'obligation' I think misses what, for me, is the crux of the matter. Just as in a good D&D game, in life you get to make choices that have consequences. If a player is not using all their PCs capabilities, you can decide if you want that to change or not, and, if so, more particularly what you want the outcome to be. Feelings about being obliged, or, on the other end, some feeling that you 'shouldn't have to' help the player are, if not totally irrelevant, at least way down the priority list, IMO.
A more interesting question is, if you do want to help the player, what is the most effective way to do it? One thing it is probably good to keep in mind in general is an important rule of thumb for giving advice: always give advice just before it is needed. In the case of D&D PCs, one could interpret that as prompting during play at the point when a player should declare the use of a PC capability. My instinct, though, is that the best time in general would be a short time before a session. The reason is that the behavior you are trying to promote is not just the player using the PC's capabilities, but rather the player thinking to use the capabilities. If the player knows the PC's features, but is just forgetting to use them, I do it just before the session. If they need to brush up on the features, then maybe the day before the session.
Prompting during play is really more modeling the behavior for the player than enabling/helping them to do it themselves. I would only use modeling as a starting point for very inexperienced players, younger kids, etc.
A corollary, by the way, is that one of the less effective approaches is talking to the player right after a session. It might work, but there's a greater chance that the player will just feel bad (even if you are nice about it) and/or the memory will fade than if you give them the advice closer to the point that they can use it.
My 2 c.p.