[JulianK]

Always advisable to get a model release form. I wouldn't shoot a model without one.

Here's a little example of why, if you're professional, you must have them:

A while back I shot a proposal set for an eye wear brand. They gave me the contract. Then I shot the actual ads. However the eye wear brand accidentally used a shot from the proposal set and posted it on a 20 foot tall ad on Oxford Street.
The model tried to sue me. I had a release - so she couldn't.

Mistakes like these happen and if you don't have a release, you're leaving yourself open to potential legal problems that could occur via a 3rd party.