Really with tub adhesives, when you grout early, you're stopping the adhesive from drying out well, as tub adhesives are water based and need air to cure.
If you do grout the same day, on a small area, try not to bed out the tiles too much and grout at the very end of the job. Try to leave the last area you tiled and stick your tools on your van and have a brew then finish the last bit.
You'd need to make the customer aware of the fact that they'll need to buff up the grout the next day.
Really though, I think if it says on the tub 24 hours, you're supposed to leave it 24 hours. It doesn't matter if it's a meter or a million meters, the adhesives doesn't dry quicker in smaller areas.
If you can use a cement based adhesive on the job instead, you'll find that grouting that early will have minimal effects on the adhesive.
