This is a really interesting thread and I'm going to respond properly, later, but I wanted to make sure I'd remember.
I would say that if you want to be a completist & it makes you happy & doesn't interfere with anything, then go for it. Some people collect really successfully that way and it's very satisfying for them. But if it's overwhelming because you feel like you
have to collect a certain way because other people do, or for some other reason, then try to just collect what you love and remember to have fun!
As for how I collect, I'll need to be a little more awake to formulate my response properly (but I did ask a similar question to my Flickr friends, once,
which you can see here if you like!)
Short version: I just collect what I think is cute but try to keep my biggest collections to Barbie (vintage & 80s), Tammy & Blythe. Unfortunately I don't display most as I've not got the room but I bring them out for photos.
Long version: I will think about it (as in, how I balance that).