From my experience at Walgreens, it is really difficult to get fired. The only reasons I can think of that would get you fired without several warnings would be stealing or failing a drug test. Even failing a drug test will not get you fired in most instances I have seen.
Try to make a good impression at every store, and make friends with the staff. This will come in handy later. Try your hardest to meet each patient's needs. They might say you were helpful to the staff later, and this will help build your reputation.
To be honest, most stores are not expecting much from floaters. I know I am overjoyed if I come in to no major problems or irate customers that could have been prevented. Bonus points for making adherence calls, checking in controlled substances, and volunteering for immunizations when two pharmacists are on duty.