Hi,
I just did the rounds on this myself.
For a frameless shower screen it is necessary to finish the tiling before calling for a measure and quote as they will want to find out if the walls and floors are straight and plumb.
If they are not, and wavy and out of square, the installers will need to decide with you if the glass will need to be custom cut ( MUCH extra cost) or if you are happy with an uneven joint line filled with silicon sealer. So if it is not too late, exert pressure to have very straight wall tiles where the glass panels adjoin.
Then if the shower screen door is fixed to the wall (it usually is), they need extra timber inside the wall to fix the heavy hinges to, usually 100 mm total width to the full height of the glass. If you have already retiled without that, it may restrict you to a semi frameless type of shower screen. These are cheaper anyway because they use thinner glass panels. If a brick wall, no worries.
When tiling the areas where the glass panels abut the wall, the tiler should butter the whole back of the tile with adhesive, and not use just dobs or pats of adhesive at each corner of the tile. This is to ensure the tile does not crack when they tighten down the bracket screws holding the glass screen.
Don't reject the use of glass quarter soap shelves in the corners of the shower recess if they suggest these. They are more than a shelf, they are actually stiffener braces to stop the glass panels wobbling.
There are a lot of imported Chinese shower screens here at the moment. This is OK if you like the look of them, but I would not buy one myself, as I think the glass may not be safe. You can usually pick these as they have an ugly plastic edging strip on the glass, and no variations at all are allowed in fittings.
Cheers