If you're just using it as a general lubricant to ease assembly and stop rust, then it's probably OK. It's not really suitable for use on actual moving parts, bearings etc.
What about using it on bolt threads that are very close to a heat source?
I removed the rocker covers yesterday to check the valve clearences. While doing so, I sheared one of the small M6x15mm cap head screws that hold the cylinder head guards on. When putting it all back together again I coated the threads with loads of Copper slip but then wondered... because the head guard bolts all screw into the head, will the heat turn the copper grease into Araldite?
Not in my experience. The grease will melt away probably but it's the particles of metallic copper that act as a lubricant and they'll stay on the threads.
Agreed. It's "standard" practice to use copper grease on those big finned exhaust header nuts on airheads. I undo mine annually, clean up, re apply copper grease and assemble.
Being exhaust components those babies get pretty hot.