Unless you develop them chances are your traffic level will never increase, so what's the risk? Not much. You either stay at the same level or start to build something of value.
If the lander for any traffic domain is well optimized there always is a small risk - for a small traffic domain - than income may drop at the outset since optimized domain landers are quite good at "getting the ads clicked". So long as the domains traffic IS targeted and the ads are relevant the advertisers are happy.
I'd say develop them at you leisure, making whatever coin you can on the undeveloped ones from parking whilst you work your way towards developing them too.
Thanks for the reply! I'm not too concerned about losing the parking revenue, its not much. I'm more worried about the time and effort it will take to develop vs. just sitting on the names. But you have a point, I doubt the traffic will increase much over time. I think I'll start with one name and see how it goes.