I obtained the rights to reuse some figures from other journal articles on my thesis from the publishing company. Is it sill a good idea to email the authors of the figures, do I need their OK before I include it in my thesis ?
In the US, you are legally in the clear if you have permission from the copyright holder, which is likely the journal. That being said, it would be polite to ask the authors, too.
Both of those being said, you probably have a fair use right in the US to reproduce the images verbatim in your thesis for the purposes of commentary, critique, criticism, or the like. However, fair use is a defense to copyright infringement, so if you don't want to run the risk of getting sued and having to defend yourself, getting written (including email) permission to use the image from the copyright holder is the best strategy.