That only means your development environment for OptiX 3.8.0 should use a CUDA Toolkit 7.0 at maximum, otherwise OptiX wouldn’t be able to parse your compiled PTX code.
That R358 display driver contains a newer CUDA driver which supports programs created with the CUDA Toolkit 7.5 in addition, but it also handles programs written with CUDA Toolkit 7.0 and earlier (down to some minimum version).
Means you can program OptiX programs with CUDA Toolkit 7.0 and they’ll run on that newer driver. Even better, if your applications are run on a Maxwell GPU that R358 driver should show improved PTX compile times for the end user.
The other way round doesn’t work though. There is a minimum display driver version required to run OptiX programs which depends on the CUDA version with which an OptiX version was originally built.
You can find the necessary information inside the OptiX Release Notes which are a mandatory read before setting up a development environment for any OptiX version.