No, backpacks are a terrible idea IMO. My college commute is 35ish miles and I use a hippack and leave books in a locker at school (usually). You end up really comfortable and with a ridiculously nasty back.
A rack would be best but traveling light is ideal... assuming you're not riding cross-state into desolate areas. I have at least passing familiarity with Valley Forge and the extensive rail system around there, so I would never be left stranded in a 911 emergency. Nor would I ever run out of water/food. So I don't worry much and carry $20, cellphone, a few tools, and water.