You can do a CMA on rentals just like you do a normal CMA in Matrix. If this is not familiar to you, take the CMA class at the Northwest MLS.
For a CMA on rentals you just choose Active and Rented statuses with the Property Type as Rental. In the price field, be sure to watch your decimal position. Rent of $1500 is $1.5 in the Price field.
I like Rentometer for a general, average number, but I find my own CMA to be more accurate. I also like the site http://www.finestexpert.com and feel it is a good double check to Rentometer and gives extra details that I find helpful.