This depends on your needs.
I'd suggest that you read the release notes of 7.5 SP4 / SP5, and then you would have to decide if you can live without the fixes/additions.
I personally recommend to upgrade to the latest SP from time to time, to stay up2date.
Now, that EV8 SP1 is out, there is no need to wait longer before going to 8 ;)
Cheers
Michel