BTW, I asked my tile sub today about the step marker tiles and told me they just inlay the step marker tiles directly into the plaster.
I think its really hard to be detail oriented as a person and have anyone do work for you. I know when I tiled our kitchen I obsessed over gaps and got very upset where I couldn't fix them, or I had tiles that I couldn't get to cut properly. I hated laying glass tile. 2 years later, I can't tell you where those imperfections are. If I spend enough time looking I can find them, but the point is, I just look over at my kitchen and smile because it looks fantastic. I don't see the imperfections, they don't distract me. If I were doing it for a living, I think I'd have figured out by now when the gap or unevenness is really an issue and when it will be hidden in the overall picture and when you do it for a living you have balance time spent over details that won't be noticed in the long run. The tile guy will definitely remember the pools he did where he left a gap too big and the homeowner made him go back and chip it all out and start again and want to avoid that. So unless he's brand new at this, I wouldn't be overly concerned.
My guide for when something can't be lived with, is if after at least 24 hours I'm still obsessing over a detail and won't let it go, it has to be redone. I have an issue where they are going to be recutting some of my coping and making it meet evenly. It ate at me over a whole weekend and I knew I'd never not see it. It was like my personal FEDEX arrow. You should sort of be able to know what that point is for you. You'll know in your heart when you have to make them redo something.