1970s house. Artexed ceilings. Hairline cracks have developed on the
ceilings, where the plasterboard panels meet.

Polyfilla and others sell "No More Cracks" filler or similar for this
problem.

Question: Would it be OK to use tile grout to fill the cracks? AFAIK
grout is basically a thin cement mixture with some flexibility.

Or should I just leave the cracks -- they'll open up again if I fill
'em!

Bruce