Right now i prolly have about 15 domains that i dont even have parked anywhere (other than godaddy defaults) since you can basically add as many domains (or enough) to dreamhost or 1and1 should i put them there just for the hell of it with some ads on it or is that worthless?
If you plan on using them at some point in time, I would throw some sort of content on the site. This way the sites should get indexed and when you do finally develop those sites, they will have aged a bit in the eyes of the googlebot.