It is good to keep drives at 60% or less full. How often you need to defrag depends on the usage of your drive. The latest OSs do it automatically.
I do a lot of video editing, mainly very large files, and it helps to make sure you have nice large clean areas for these.
I still manually defrag, especially when I have cleared off large project files. Our operating systems have become much better at handling things themselves though. Unless you totally stuff a drive to the brim things don't tend to grind to a halt like they might have in the past.