Yea, the whole "strength when i need to" thing is something i have been working on and thinking about. I find if i try to use strength/speed all the time then i end up giving more space to my opponent and i get tired quicker. And i my conditioning is bad enough what with my Starting Strength/mass gain and all this eating. But if i go slow and settle where i need to, like top position, i find i can roll more technically and use less energy. I save the strength and speed for passing guard or escaping from bottom during a hard match.
A good example is the other day i was rolling with a newer guy, at first i was relaxing and just trying to play a technical game, i was doing well, holding top and generally not expending that much energy. Then time went on, i started to think i had to sub him, so i moved quicker, started getting more tired, he got his guard back, and eventually we just stopped because he wanted to join another class. I think if i had concentrated more on being technical and not just about "winning" that would have been a better course. Something to work on.
Then there are other guys my own level and above that when we roll they bring it, and its going to be war involving technique, attributes, and conditioning.
I think both kinds of rolls are good and necessary.