So for security reasons, I'd advise keeping the bulk of your funds offline in something like a Tresor or a paper wallet. Personally, I split my coins into "silos" on different "offline wallets", with just my trading funds in an exchange, and some "hot coins" for paying for things in a local wallet. Having been a victim of MtGox in the past, I don't trust exchanges with much ;)
RE: Where Do you Buy YOUR Bitcoins???-.- Community Open Questions Program