As far as I know, you are not REQUIRED to install Lightrooom CC when you buy the special photography plan subscription
However... you may loose any integration (open with) features if you don't have both programs on the same version level
Since I don't use Lightroom, I can't say for sure that you WILL lose any integration features
If you have the Photography Plan you would be best to install LR CC 2015.4
You will then get the benefit of the latest features such as the Dehaze tool in Effects and Boundary Warp with panoramas. Adobe only provides bug fixes and camera raw updates for LR6 whereas subscribers receive all new features as they are released. You could of course use those tools in Camera Raw, within Photoshop CC, but your images would not be compatible with LR6.
thanks 99jon - you are getting at my real question - if I sign up and use the photograph plan and later quit for whatever reason, will my copy of LR6 manage and edit my then existing images - both pre photograph plan and post plan images? thanks
Yes, except any new features you use, e.g. Dehaze slider, wouldn't be recognized. But LR CC dosen't stop working if you stop subscribing. Only Develop gets disabled. You can still use Library to export, and for Quick Develop. All the other modules also remain functional.