Hi Julia,
Without seeing these sites you're referring to it's hard to give you specific advice or input but what I will say is - if there is a fee be sure you do your homework before paying up! The VA industry is fraught with scams and so many sites will charge you monthly just for access to a few RFPs or reconstituted listings from oDesk, etc.
Of course, if the networks are free then there's nothing wrong in checking them out
This isn't to say that all paid sites are bad, just to advise you to be very very aware of scams and be careful about who you give your money to and what you're getting in return.
Good luck