Its good to work your muscles from different angles to place different kinds of stress on your targeted muscle. You'll grow quicker if you change things up every once and a while.......Don't let your routine become routine. Standing barbell curls and alternate curls work the upper and overall bicep, seated incline alternate curls kind of work the middle, and preacher & hammer are good for the lower portion....as you know. I think you should always do one of the compound exercises for biceps followed by a couple of others. Just try to use as many angle as you can.
As for your other question, I really don't know anyone that has only worked their lower biceps. lol. Im thinking it might look weird.....but i really dont know. I'll leave this question for someone else.
I dont know why you would just want to work the lower portion.
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~If your not going to give 110% at the gym then why bother going at all~