Newcomers to recording are often seduced by the 'smoothing' qualities of reverb and add way too much of it. Experienced recording engineers often appear to use no reverb at all. Somewhere in between there must be a happy medium.
But first, consider what the purpose of reverb is. Largely, it is there to compensate for close microphone positioning. If you record an acoustic instrument in a hall with good acoustics, then with the mic in the right position you don't need any artificial reverberation at all.
But the sound of close miking in popular music is very attractive to the ear, so if a little bit of artificial reverb has to be added to compensate for the lack of natural reverberation, that is an easy decision to make.
The wonderful thing about natural reverberation however is that the ear doesn't notice it. Natural reverberation is all around us in everyday life so there is no reason for the ear to be particularly interested in it.
For most purposes, we should aim to do that with artificial reverb too. It should be applied in a way that doesn't draw attention. Yes, like natural reverberation it will be audible if you listen for it. But unless you specifically concentrate your attention on the reverb, it should not be particularly apparent.
So the correct amount of reverb to apply to a lead vocal is just enough to compensate for the lack of natural reverb due to the close microphone position. If you can hear the reverb as a sound element in its own right, you have put on too much.
Of course it would be incorrect to say that this is a 'rule' of recording. There are very few rules in recording that you absolutely have to abide by, and this isn't one of them. But as a guideline it is a good starting point.
Now - background vocals. The usual practice with background vocals is to take steps to place them distinctly in the background, of course, so that the lead vocal can stand out.
You can do this by lowering the level of the background vocals, EQing them so that they don't stand out so much, or you can apply reverb to make them more 'blended' than the lead vocal.
So, often it will be appropriate to add more reverb to the background vocal than to the lead vocal.
Here are some examples of reverb applied to a lead vocal. Of course, it's just my opinion on what is just right or too much, but it should put things in perspective. The singer is Moira Rumveye (with a cold and a Neumann M147 microphone)...Come on the FREE COURSE TOUR