You're talking about Coinbase right? Well, theoretically it can happen with any exchange and I know of at least 1 case on Kraken. However, I would recommend Bitstamp. I've been using it for years and have never had any problems. As long as you don't act 'suspiciously' you should be fine.
BTC-E the exchange I like to use to trade bitcoin never had a problem.
I have not heard of them closing accounts but you would have to double check.
How can I buy BTC from these services ? (BTC-E and bitstamp), I understand BTC-E no longer accepts bank wire/transfers from the USA, bitstamp however still does. Are there other payment methods accepted by these two?