I would remove the valves, pull off the olives & nuts and drill an 18mm hole in th tile and slip it over the pipe rather than cut a notch out of the pipes.
This leaves a small neat hole that can be filled with mastic thats colour matched to the grout and will allow for any expansion as the pipes heat up. You should still be able to mastic around your pipes though if the gap is large then pipe covers may be the way to go.
I covered mine tile cuts with those crome pipe rings. Its like a crome washer that you split, slide down the pipe and sit it over the hole where the pipe comes up through the tile covering the hole and an area of about 10m all round.
But thats cos i'm not as professional or neat as JasonB
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