I love being so old I remember when singers had to sing in tune. I'm not suggesting that everyone used to go into the vocal booth and hey presto...there was a spine tingling vocal performance completely in tune...far from it. You might find some real horror lurking on tape, or you might get your mind blown - for reference see any early 90's R&B backing vocal stack. Trust me, even us rock kids were secretly listening to En Vogue and going wow. It just put the emphasis on the performer far more. And that's another thing - performance. Maybe a performance is better with those imperfections, character, out of tune notes here and there. Sometimes it is, and sometimes it isn’t. Before all these cool software tools came along, we'd slow down the tape to allow people to hit those tricky high notes, to nail those harmonies (it sounds way cool actually, sometimes - try it if you have a sync unit that will let you ) drop in and out a hundred times if need be...even a bit later sample and re-pitch things if necessary, then fly them back in with a button press on a sampler by hand hoping to get the timing right, or even for the truly brave or technology deprived - just fly it from another tape machine - meaning press play and record the part. Us studio weasels have always used our tricks and toys to make reality better than reality. And then again, we, or the producer would use every trick in the book to get that performance from the singer. I was shocked the first time I saw a young singer literally cry as she was put through her paces by the producer I was working with as he made her do it over and over and over to get what he wanted. Pitching, phrasing, timing - nothing was overlooked. I really had to stop and think about the level of my OWN quality control when it came to recording at that point, as this was another planet. As an engineer you get to sit around a lot and pay attention to the whole picture and you see both what someone wants to achieve, and also have to deal with the singer whining in the kitchen later. Brute force is one method, sure, but singers are delicate creatures with egos the size of planets, so best to find other ways.
At some point, tuning software showed up. Autotune came....was abused...became totally uncool, then...came right back again and here it still is. There are lots of similar tools out there by various vendors, and of course things like Re-voice and Melodyne, which offers a more precise and less invasive option when it comes to tuning at the cost of actually having to do something. You can probably tell I love it. If you want to do a good job however it's not enough to put everything in tune using default settings, as many singers bend notes and your new right in tune vocal according-to-Melodyne vocal may now sound...weird. This kind of effort is what will separate you from the other kids. Attention to detail.
My advice in general is to follow a "do no harm" approach. Do not tune by default unless the production requires it, and even then be more careful than the next person. It's not really my job as a mixer to make that kind of decision and you'd like to assume that the producer took care of it, so you have to make a judgement call on when it's appropriate. There are even people out there who just do THIS for a living, vocal production - so there's another option for you if that's your bag.
So when semi dicey vocals arrive, the call here is do I have the time to wait a couple of days for the emails to bounce back and forth, to wait for new files, to incorporate those or do I want to make that clever harmony part at the end that sounds like dogs wailing accompanied my some well intentioned but out of tune guitars sound like music just because I give a crap about the job? I know what the artist was AIMING for, so why not help them get there, do it, and not mention it?
Of course, you will get those comments back sometimes to "remove the Autotune" when you didn't add any, or to retune everything now they can hear everything clearly so always be prepared to recall what you did, or didn't do. The easiest way to do this is to make a copy of the whole lead vocal playlist untampered with BEFORE you start fixing vocals so you can grab sections if need be and pop them in and get back to where you were originally without having to open some ancient version of the mix. Tweak that which needs to be tweaked and leave good enough alone!