I have inflexible knees in my opinion. When I stretch by putting an ankle on top of my opposite knee, I get a good deal of discomfort on the outside of my knees.
That being said, I worked a bit of rubber guard last night without a hitch. Worked basic path with no problem (failed the omoplata sweep, but that's not the point hehe). No pain. Not even a feeling like my knee was being stretched at all.
The way coach taught me, you don't need a whole lot of flexibility to do RG effectively. On top of that, he stressed the importance of hugging the knee. Now I'm a total noob when it comes to RG (and JJ in general), but I think it's worth considering that hugging the knee might be a big contributor to protecting your knee. Hug that knee tight and you should reduce its mobility and I'd assume you'd prevent injury, no?