Storing coins on Exchanges

Hi,

Longtime lurker, but finally registered as I can't see an obvious answer to the below question.

I've got accounts on MtGox and Bitstamp (although no longer use Gox due to known issues). I don't have a sizeable number of coins, but luckily the value of these is going up all the time, so I really should think about security. Is there any reason not to store coins with the exchanges? (aside from the obvious exchange goes bang..) I'm adopting the possibly naive 'they know more about this than me so their security will be better' approach...

Also I'm sure there's a step-by-step guide to how to get coins to a paper wallet - would anyone kindly share this with me? Nb - I have a blockchain wallet already set-up but haven't used this much.

Many thanks in advance,
Bingo