i have quite a few piercings, including nose, lip & tongue. I dont feel like i should take them out to do a job, though i feel that like
lynlockwood put above that it does depend what job you are applying for, i have worked in a music shop and clothes shop and neither of them minded my piercings.
Try asking in the interview as said above, they should tell you whether they are okay with you wearing them.
What i have done in the past with a job in a shop that werent too happy with my lip piercing was get a see through retainer for it, like this one:
http://img.hottopic.com/is/image/HotTopic/2118 38_hi?$product$
because i didnt want to take it out and have it heal up.
I totally agree with what your saying. i always think what does it matter if you have a bar through your lip, it doesnt affect your personality or how polite you are! Though are are some people who seem to think it does!
If somewhere is not happy with piercings being in at work, rather than turn down the job because of that, maybe just get a see through stud for it like i did instead. :-)