Trim should be conducted by windows trim service either way on windows because of wear leveling in the service... as the virtualization software attempts to rewrite a page that has been written, windows will change the lba of the page to a different address that has been written less times. Where you will see loss in performance is if the drive remains full for a long period of time while writing small intersections into the remaining free space repeatedly. This is where the wear leveling fails as it will usually not move already written blocks, that don't get rewritten often, to worn down blocks, so that the fresher blocks can be used more aggressively.
With that said there is a disadvantage to having your os on the HDD's because the page file is usually on the disk you installed on. In the case that your machine begins to thrash, you will see a massive degradation in performance. A likely scenario with 8 GB of ram running VM's. I would suggest that, if you decide to install on the hdd's, you at least move the page file to the ssd and increase it's size. That way the spinning media will cache items to the ssd as they are unloaded from ram and be more accessible when needed later.
BMDan is correct in his suggestion though, having the os on the ssd and then only using the ssd will considerably reduce power consumption. Depends on how much space you need.