If you didn't meet any problem, you need to decide yourself, but usually, when using Windows for some tasks only, there is no need to increase CPU and memory load for VM
I only use Parallels and Windows SP2 (on Tiger Mac Mini) to run one program... Winflex... a quote system for life insurance products. I don't run it very often. What would SP3 do for me? Anything? Security? If I run the update do I need to do anything to Parallels?
There are security updates and bugfixes of course, but using SP3 increase CPU and memory usage, so it is up to your decision.
You can try to install it , after backing up VM as described in Parallels Desktop for Mac user guide from http://www.parallels.com/en/download/file/doc/Parallels_Desktop_for_Mac_User_Guide.pdf on page 243
If there will be problems, you can return back to SP2