There are two key things that you should consider:
1. Why do you need 5 minutes to solve a question that should be solved in 3 minutes or less. Do you lack some core skills or are you not solving the problem in an optimal manner.
2. Are you running behind time/on time/ahead of time. If you are running ahead of time, then you may spend the extra time , otherwise if you think that you are going to take longer (and you are already behind) then go ahead and skip.
I address questions such as above in my free session tomorrow on tactical strategies. Click on the link below (or register at e-GMAT
if you want to receive a reminder). http://gmatclub.com/blog/2012/07/gmat-s ... -session2/