Two macs...one login?

I just got my new MBA. I already have an iMac (plus an old MBP). The question is should I use the same login for my three machines? I was thinking of using migration assistant to move what I need (Mail, iTunes, etc) from my iMac to my MBA. THat's what I did when I bought my iMac seven months ago and I transferred those libraries from my old MBP. I can't remember if I had the option of creating a different login. Does it even matter?

I intend to use my MBA mostly as my travel machine, especially on trips when I'll be taking lots of photos (I'm an aspiring photographer). My main Aperture library will be on my iMac. When I get home from my trips, I'll be exporting the projects on Aperture back to my iMac.

Thanks....

 

Sorry I should have been a bit more specific. I'm not concerned with security. I'm more concerned with what happens when I connect the two machines on the same network and try to transfer files back and forth. I don't think there should be any problems as both my iMac and old MBP are now on the same network under the same login but I've never tried transferring files between the two...