Everyone is different, of course -- depends upon what software you have and how extensively you've done work in it which results in files to keep within it. Prior to the last time I went through this process, I looked at my applications on my iMac and decided which I wanted to go ahead and put on my new MBP. Most were Apple Apps purchased through the Mac App store, so easy enough to simply download them into the new machine at no extra cost. Most of my other software programs were either free and easy to download again into the new machine or were ones I was no longer using so didn't need to bother with at all.
I also need to clarify here: I actually handle doing backups manually -- instead of using Time Machine or SuperDuper, I plug in the external drive and then copy over the various folders from the source machine to the external drive -- that is: documents folder, pictures folder, movies folder, music folder, etc., etc...... I just find this a much easier way to work when I want or need to retrieve something from backup. This has worked well for me for years and I see no reason to make life more complicated!
Yes, by setting up a new machine in this way you do have to take the time to go through System Preferences and set up the machine to your liking, need to tweak viewing preferences and the like, etc., but in the long run I find this useful because while I'm doing all of this I'm also exploring and becoming acquainted with my new machine and if there are going to be any problems I am likely to spot them early on (hopefully before I've put all of my personal stuff into it). Kind of a "bonding experience" between me and the new machine.....LOL!
Not long ago a friend bought a new iMac and we lugged her old iMac over to the Apple store so that she could leave it there for the store personnel to transfer all her data from old machine to new one. It took two days. When she had both machines back home, she wiped the old one clean in preparation to give to her brother and started using her new machine as if there had been no interruption, as everything on the new machine was just as the old one had been. She was thrilled. As I see it, though, the problem with this is that more than likely along with the good stuff, there was probably a bunch of old dreck that trailed along into the new machine, too, and I can't help wondering if at some point somewhere along the line that might cause problems, as she's still carrying around stuff that was on her old Windows machine and her first (PPC) iMac in addition to what was on the last iMac. Whatever.....
