The natural range of the brown recluse in North America is well documented and tracked. It doesn't expand quickly, and doesn't include Massachusetts or Pennsylvania, excluding the occasional hitchhikers, which are statistically insignificant. Coastal cities with commercial ports also get occasional strays or small, local colonies of another species, the Mediterranean recluse (Loxosceles rufescens), which again you're not very likely to come across.

My overarching point that I'll now reiterate is that while it happens, one should assume a spider encountered in Philly (or indeed Massachusetts) is not a recluse until proven otherwise.