Good question. I am assuming you are specifically talking about closed guard. I have been doing much more squatting and dl'ing in the past couple of months and have noticed a slight decrease in my flexibility. Well more like I have noticed more pain from when someone is putting me in a difficult position like trying a half guard sweep or a leg over guard pass.
My closed guard I believe has benefited from the additional squats and deadlifts. Here is why. I don't play the rubber guard ala Eddie Bravo unless something bad has happened and I am using it to try and recover butterfly or closed guard. Like here.
I had butterfly and went for something and he got a hold of my left foot and I was using this position to work for something else. I have never trained rubber guard and really doubt that this is anything more than me holding on to what I had.
But in closed guard, especially if I get control and get a higher guard I can really latch down and it is very difficult to get my legs free for my opponent to pass.
So I guess it depends on what you are more comfortable with or what you are working on at the time. I know that it takes a lot of flexibility to really work Eddie Bravo's stuff.
The main thing I have worked on this year for closed guard is to always be attacking. Either a sweep or a submission. No matter what attack. It has seemed to work for me.
I really look forward to hearing what everybody else thinks. By the way I just laid down on the floor and can not put my leg behind my head. At least cold anyways.