CONGRATS on getting them spayed! Hooray! I am sure they will do just fine! But pre-surgical bloodwork is a good piece of mind & also can alert you of any problems that may affect your dog. They test for multiple things (depends on the actual panel you or your dr. chooses) but it ussually includes heartworm test (although a seperate test, should be performed on all dogs especially before anesthesia),they pretty much make sure all the vital organs are working properly like the kidney, live, panceas & thyroid, it also ussually tests for glucose & electrolytes.
here is a good site w/ some valuable info:

http://www.thepetcenter.com/pha/cp.html

good luck & again thamkyou for being a responsible owner!!!!