As for UPS,
I hope I can explain this (from what I understand) and not sound too confusing. UPS likes it if you have other experience other than flying. (Especially throwing UPS boxes or delivering UPS boxes) They like to see that people have struggled or sacraficed to get where they are in life. That being said, I think if they know you got a job throwing boxes for the sole purpose of getting ahead for a flying position with them, they might not like it. Ive never heard anything official regarding this, but knowing what I know about the relationship between pilots and "others" in the company. (IMHO) It seems like they wouldnt like you using "their jobs" to get one of "our jobs." (us and them mentallity)
There is another side to this, Say you end up working for some boxhead punk who is you supervisor. He/she doesnt like you because he knows that one day you are going to be a pilot making 5 times his salary and he will still be a boxhead throwing packages and gives you bad evals or gets you fired... Then you will never get hired here...
This is a great place to be, however there is a large devide between the pilots/mechanics and anyone else that works here.
My advice (if you want it) focus on flying if flying is what you want to do as a career. Study everything about UPS/FedEx and know all you can about the company before you get the interview. If you have extra time, work on getting the interview (connections/LOR's/Etc)...
Again, this is speculation, Ive never heard anything official from UPS regarding this. I have met pilots here that use to throw boxes for UPS BEFORE they started flying, Ive never met any that got a job throwing boxes after they were a Professional pilot...
Good luck
BigbrownDC8