I have to agree with the wording that Brent used "take the practice test early in your studies"
In other words, I think it is a good idea to at least find out what, for example, Data Sufficiency is all about and do 10 or 20 of those problems before trying the GMATPrep Test.
Maybe just do the 15 sample problems on each topic that are found in the "practice content" of the GMATPrep software.
I feel like if you take the test without any idea of how to do the questions then you will get an artificially low score. There is still plenty of room to improve with studying, but it does not make much sense to attempt the test if you are not familiar with, for example, the answers choices on Data Sufficiency.