What ever you do with your first models...Take your time!!! Try and understand exactly what you're doing and why you're doing it.
I found it all to easy to follow the tutorials step by step, but at the end you think to yourself...Why did I just do that?
The cartoon dog is a very good one to start with, also the basic human. I've done both now, but I had more confidence with the second tut. I'm busy doing the Chef Ramsey tutorial where I tend to watch the tutorial section then sit down and do the work. I also try and add some of my own ideas, so that I learn a bit by myself. Don't worry about time at first, like everything, practice will bring speed.
What has helped for me is saving often with a new file name...That way I can re-trace my steps if I've made a mistake. Oh, and set your undo limit to 99 or infinite!
Finally, post your work! It's good to get feedback, both good and bad. That is how you learn! I still need to post my cartoon dog.
Happy modeling!
-Jacques
Poly-pushing on a '96 SGI O2