I would use a 1/4" grout line, myself.
The only rule I follow is make the customer happy. Lay a few tiles out
on a plywood with different spacings, then decide which one you like
best. You can stuff toilet paper in to simulate the grout.
In any case, seal the grout when you're done. Your tile store can
advise you on what's best for kitchen, bath, etc.
You are gonna use a tile store, right? The few extra bucks you pay for
supplies you'll more than get back in support and advice. Besides, they
might even do the grout line mockup for you.
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