You can also use snippets. Create a single snippet with signature for everyone. You can use segments, like Josh suggested, on snippets too. The only thing you would have to do is add the snippet to your email footer and never worry about it again.
Segments you could use:
- Industry, when you have marketers or account managers or business units with specific industries.
- country (or state), when you want a different signature per region (e.g. different office address details).
- language, when you want the signature in the preferred language of the lead.
You could also look into lead owner vs account owner.