Boost dependency for a C++ open source project?

It all depends on the way you're going to use Boost. As Diomidis said, if you're going to use some non-trivial facilities from Boost, just go ahead. Using libraries is not a crime.

Of course, there are many people who prefer not to use Boost, because introducing new dependencies has always some cons and extra worries, but in an open source project... in my opinion it's even alright to use them if you just want to learn them or improve your skills on them.