Were I in your position, I would adopt a slightly different stance.
On being presented with material for mastering, I would do the mechanical things (top & tailing, fades, etc), but I would be reluctant to try and emphasize something that I personally believe should have more prominence (such as increasing the vocal levels). My personal views on musicality (which I'm entitled to have) may not accord with those who supplied the material (which they're entitled to have), and I have no right to second-guess the customer.
For one thing, it is (as you note) devaluing the mixer's work (maybe even insulting it), and prima facie, their opinion carries as much weight as yours.
For another thing, what you are seeking to do is to perform an adjustment during a phase of the process which is sub-optimal for doing it. The best solution is a re-mix and a re-submitted product for you to work on.
However, I would always check with the customer: "Did you intend for the vocals to be this low? Would you like to have a listen to see what I mean?" This is a desirable path to follow, because even though you should not impose your musical ideas, you should confirm the intention behind any musical oddities. It's always possible, for example, that they sent you the wrong mix.
I can easily imagine a scenario in which you do a job unquestioningly, return the product and field a barrage of complaints from the customer: "surely you must have heard there was something wrong with the mix we sent you. It's obvious! Why didn't you check with us instead of blindly doing it etc etc".
And I can just as easily imagine a scenario where you take the mix, massage it to your own exquisite taste, only to field a similar barrage: "how dare you take it upon yourself to sit in judgement on our material and alter it? You've ruined it! etc. etc."
This leads me to two rules:
1 Never second-guess the customer. It is dangerous to assume their intentions; and
2 Always confirm the brief, specially if it involves you having to perform major surgery on the tracks.