Disclaimer: This is specifically regarding striking, and general clinching/throwing. Ground grappling is different.
I do both, depending on the circumstances. Most of the time, I don't think too much and just let my body react, and fight based on that. Depending on the situation, I will choose moments, between attacks/defenses that I want to focus on a specific technique and do that. I'd say like 75% of the time it doesn't work, because I try to force it rather than waiting for the right opportunity, but I'll keep trying it until I get it, then move on. It does a good job of helping me use what I've been drilling, rather than relying on my reflexes.