Not necessary at all, but as long as you have some experience, you should be OK.
Most of my friends who got into school and did well did not have direct work experience, but other clinical/scientific experience (bench research, project work, etc...)
Having a job is not necessary but shows that you can handle juggling between school and work (while maintaining a good GPA) and also work ethics. That said, I have zero actual work experience. XD I only volunteered just like you did and got interviews at all the schools I applied to (only 2). I think it helped that my GPA was above average. So no sweat! There are plenty things the interviewer can talk to you about besides flipping burgers at McD.
I know for some schools they specifically state on their website if they prefer pharmacy work experience or not. Make sure you check the schools websites! But if you have a good GPA/PCAT scores and volunteer experience, I wouldn't sweat it too much.