When I design a logo, can I use a font that was included in my OS? That I downloaded for free? I'm perhaps less concerned with wordmarks than with other types of logos.
When you purchase or download a font, there is usually a readme file or a license included which states the usages the artist declares safe.
It might be free for personal use, but you have to pay for commercial or even just acknowledge the creator's work.
Look at the license, it will answer all :)