I guess it depends on what you're looking to get out of your practice sessions. I think the important question to ask yourself is this: what are you trying to get out of your sessions?
If you're trying to loosen up when it comes to drawing, and potentially work on your speed, then I've found Class Mode to be very helpful. It forces you to do more drawings faster, forcing your mind to work on seeing the "bigger picture" and focus on gestures before you can move on to filling out more details.
However, if you're working on a specific element, using timed mode could be a lot more helpful. If you're looking to work on your anatomy in different poses, for example, then your method could work just fine for you.
Really, the most important thing is to know your goal when you set out to practice, and the more specific the goal, the better. Drawing more in general does help, yes, but your brain learns better and retains more if you give yourself more focus. Then, based on that goal, decide which one works better.
