When removing old tiles how much of the old grout/addhesive needs to come off?

[Dozeynoo]

We are having a new kitchen fitted next week. We are having the tiling done by a separate contractor. We have taken the old tiles off before the new units go in but don't know how much to time to spend sanding/removing down the old adhesive - we have spent some time sanding it down but you can still see the grooves in it. The grooves are down to about 1mm.
We can't ask the tiler as I haven't chosen one yet - a bit of a failure on the organisation front!

Thanks.

It'll be fine so long as it is sound.
The tiler will make the surface level with the adhesive.