Should I accept a less-preferred offer while waiting for my dream firm?

A few answers. I should note that my responses are highly subjective and relative. Please take it with a grain of salt, and I trust that my colleagues will provide some counter-points to balance this out.

1) I would personally take an offer I am less satisfied with. Right now, the saying "one in the hand is worth more than two in the bush" has never been trueer

2) I think that, in one's career, you have one, maybe two (if spread out) chances to go "off the books". Again, in this case, if I had done step #1 and then, a few months later, were offered the dream job (and I was unhappy in my current job), I would switch.

This being said, you can only do this once AND you have to do right by all parties (i.e. give truly advance notice, be honest about your reasons/dreams, give company #1 time to adjust to you leaving, etc.).

People generally understand if the unexpected comes up and you have to follow your dreams/heart. However, you have to do it in an honest way (i.e. you're not taking offer #1 when you know offer #2 is coming in 3 months)