Opinions will vary, but as long as you haven't fully fitted the cooler and the thermal paste is okay then its fine to re-fit it.

It doesn't matter if the CPU has been powered, the issue is whether the thermal paste will still function correctly having been disturbed when you removed it. If you're in any doubt I'd remove, clean and add more paste.

 

I would replace it, although you probably wont see too much difference in everyday use. more of a peace-of-mind thing.

 

You should definitely replace it, once the thermal paste spreads and you take it off and put it back on there will be lots of air bubbles trapped between the chip and cooler.

 

Agree with slh28, if you've attached it - as in made contact between the cpu and the hsf, and then removed it, its screwed up, you'll need to do it again.

 

If you still have an issue look into low profile ram from corsair

 

You can run the computer with the old thermal paste in place, but your temps will start out a little higher than they could be. Over time the air bubbles that were trapped in the paste will expand and eventually push the paste out from between the heat sink and processor causing the paste to pile up in some places and leaving air gaps in others. Your temps will begin to rise very quickly after a few months to a few years. I usually replace the paste every time I remove the heat sink, but I have obviously gotten lazy at times.

 

I would replace it. Not that much work involved, anyway.

 

Do the right thing not the easy thing; builds character (whatever that is)