Assuming you get your PVT in around 50 hours, you then will burn up about another 40-50 to get your Instrument....that gives you an estimate TTL 100 hours, with PVT & Inst Rating.....
Then right after that do your Multi-Engine, (you will do 1 check ride and get your Multi Engine and that includes instrument rating with it, since you will already be insturment certified).
Now you burned up about 15-20 hours of ME time for the rating above, that's a TTL of 115-120 hours...You must have a total of 250 hours to get your Commercial, so that still leaves 130 hours.....save the last 20 hours for the Commerical Mulit-Engine Standards Practice.....so that leaves you with 110 hours to burn up.
Keep in mind you might want to save about 5-10 hours off that time to burn in training for your Commercial Single Engine.
As you can see you still have the "extra" room to get your 100 hours in after your instrument, plus you save a check ride.....Also after you have your instruments it will free you up on some scheduling flexibility issues when the weather kinda sucks....