The standard advice for any investment or trading instrument applies also to Bitcoin. Diversify your investments. Don't invest more than you can afford to loose. etc.
Even though Mt.Gox is probably one of the safest places to keep your BTC from accidental loss, you may want to keep only the BTC you are trading (not more than 10,000 btc). It's silly to keep a lot of cash on Mt.Gox (greater than $10,000 average balance) because Bitcoin will be appreciating significantly. for a long time to come. If your not speculating, don't plan to, don't want to be tempted to, then there is no need to keep any btc at MtGox. Unlike cash, BTC can be moved in a matter of minutes. I try to keep BTC spread around on 2-3 on-line wallets or exchanges. The rest I download to my (several) wallets and back up onto flash drives that are hidden. I travel sometimes to some dangerous places so Bitcoins are the perfect emergency currency. (These numbers are only relevant for only through summer 2011)
Mt.Gox has an incredible amount of potential and probably already is negociating with venture capitalists who will guide it to a huge exit (IPO, Acquisition, PE, reverse IPO...).