Hmm. If I were you I'd be pretty unhappy. Quality of build seems to be more important than the rims/spoke/hubs. A well built wheel should hold true for quite some time (although occational truing is probably nessacery.) Granted, a $300 bike will not have great wheels, but it would seem to me as though it should take more abuse than what you described. I have a set of low end wheels that I routinely take off 2 to 4.5 ft drops; they have only needed truing once since Sept. On another bike I have Alex DH 19's. I've had good experiences with the brand, but have not tried that actual rim.
Not knowing where you got your bike I feel bad doing this, but I might question the quality of the build.
OTOH, there is something to be said for riding smoothly. Maybe, you are already a good rider, but I know I thrashed my components a lot more when I first started riding.
So, to actaully answer your question: i would take it back to the shop a and get it trued since it would probably be free. Then I would try to find hardcore bikers in the area and ask them if they know of any local, really experienced wheel builders. If you continue to have problems go to him/her.