well it depends on what processor you have. If you have a northwood then you have no need to upgrade. If you have a prescott the I recommend upgrading to A05. I would not recommend upgrading to A06 becuase you loose the fn + z + r command to control your fans. also I read in another thread that A06 has been taken off the dell site again. I think A06 can wait.
A03 was released right before MR9800 became available. One of the features of A03 (maybe the only one) was support of new video cards. So, if you're planning to get MR9800 you'd better flash A03 or latter one.
A04 had bugs. Real choice you have is A03, A05 or A06.
Question is why should you NOT upgrade? I can understand not to A06...but as for the other versions, there could be some enhancements and bug fixes though I am not quite sure what. I am on A05 btw and everything works fine for me.