As others have already said, it depends a lot on what it is you're trying to achieve in terms of gameplay, atmosphere, audience etc..
I will add some additional thoughts, though. Aside from appealing to your key demographic's tastes, preferably there should be a reason other than "just because" - as in, a design reason.
A simple example is reloading a gun. Most videogames handles this very casually. However, in games like Mafia: City of Lost Heaven and Call of Cthulhu: Dark Corners of the Earth, if you carry a fully loaded revolver, fire off one shot and then hit reload, you lose the remaining five, because your character simply empties the cylinder. This is deliberate, to simulate the fact that you wouldn't have time to save individual bullets when reloading in the middle of a battle.
The reason I bring up this example is because, aside from appealing to the intended audience (more realism, less casual), it also serves an actual gameplay effect; suddenly, there's a small element of thought and strategy that goes into something as banal as reloading a gun; on one hand, you typically want to be freshly reloaded to have as many rounds ready to fire off as possible - on the other hand, you don't want to waste precious ammo by reloading after only a few shots. Suddenly something so small actual becomes a decision that is not entirely insignificant.
With this example in mind, allowing your player to heal when not needed doesn't immediately serve any purpose other than just penalizing those who make an erroneous button mash. What if you instead turned it in to a mechanical decision? Sort of like you can heal when at full health and consume the medkit, and it will go over you normal max health, but it will immediately start counting down towards your normal max, and after that, maybe even dip a tad below (to simulate over-medicating or whatever). This will give players a temporary boost for a tough battle, and they will make a conscious sacrifice by gaining a boost now and paying a lesser price later.
This is just one idea, but the general thing here is; try to see if you can incorporate an actual reason other than "just because freedom". Not only will it lessen the risk of annoying players, it will also elevate the small strategical options in your game and make for a more interesting gameplay.