It can be obnoxious, especially when it costs so much time and effort to go back for them. But it's worth it in the end.
Off the top of my head, with Sam I've gotten ones from Nar Shadda, Ord Mantell, Coruscant, that bug planet, tatooine, and was in the process of nabbing ones on alderaan. Problem is my roomie moved out and his comp had the game and my toon.
To put it lightly...while we're still friends, he owes quite a bit on internet overages, food he couldn't pay back, and shorting on rent several months there.
What time I did have with it I learned of 'crons in the fleet for both sides. Those are the best and also the most difficult to obtain.
I've found some on most planets I've visited. I got the two on the tatooine sandcrawler. The latter one for STR being a little more difficult to get but since apparently I still have action game reflexes, I was able to devise a strategy that worked the first time.
You line up above with where the platform is below that you fall to. Stand such that you'll face the 'cron when you fall on the platform (back facing outward). Jump and while in midair, make *little taps* backwards on the direction pad and be absolutely sure the camera is looking down on your toon from above. Line up and do it right, you'll get it. It was actually easier than I thought it'd be.
We'll murder them all, amid laughter and merriment...except for the few we take home to experiment!
"I cant see S***! --YOU GO TO HELL!" --Tourettes guy