Your laptop will not be hurt inside the bag or pannier as long as its shut down and not just suspended. I can't imagine your commute route being that rough to break the display or hurt anything else. The internals are all solid state and it is a mobile device so I would pack it. I have a new fairly expensive dell laptop from my company and it is the cheesiest, crappy laptop I ever had and I have carried it in panniers with no problems. If all you have now is the backpack, then by all means strap it to the rack, absolutely do not carry it on your back, that just puts more weight on your saddle which hurts your butt and you'll get really sweaty. You can get a set of panniers for fairly cheap that will carry your clothes, lunch and laptop. Mine does. Its a lot of weight when you add this all up but its commuting not a race.