It seems the stress should be low, right? The boss isn't usually breathing down your neck, and what's the worst that can happen? You take too long to serve someone their beer? Pretty low stakes. If you want on-the-knife-edge stress, go check out brain surgeon.
However, things can get tense. If the bar is totally packed and a few guys get drunk and into a shoving match, you might have to step in. Even when things are going normally, you have to move constantly and you are always having orders snapped at you—it isn't uncommon for a bartender (usually a fledgling one) to break down in tears or experience a panic attack. So the stress level for this one is probably a solid "5."