Any experience is useful because it will teach you something (even if only that you hate that setting). However, if you are so set on doing a residency (though it says you are pre-pharmacy... there is a big chance you will change your mind in the process as you discover all the opportunities pharmacy has to offer), obtaining a job at a hospital can be much more beneficial. A lot of the time, once you are accepted by a pharmacy school, that school's university hospital would be willing to hire you and give you some hours. Look into that option.
That's, of course, if you mean traditional hospital based clinical or drug information residency. It becomes a bit different if you are interested, say, in managed care, or industry-based, or community practice residency. For them it would be helpful to find a job in one of their respective settings (such as managed care, or industry, or VA). But retail could still be useful... for an intern, retails is what you make of it unless you are at a very busy store (upwards of 400 Rx/day)