Yes it's a risk - though if your bookie bet wins you've no liability to lose which is great! But if your bookie bet loses then you'll get nothing from the exchange...
Timeisnow - you do need to build up some money in the exchange in that case so you can start to put your free bets on at higher odds.
A £20 free bet with back odds of 2.0 and lay odds of 1.9 will only make you £10.81, whereas putting it on at back odds of 6.0 and lay odds of 5.7 will make you £17.70 so definitely worth doing those high odds 