Generally speaking load tests will run through common scenarios with various user loads.
So for example, you might set up a test wherein 50 users login every second for 10 minutes and another where the number starts at 5 users/second and scales up to 1000 / second or whatever numbers make sense for your your site.
edit:
The idea is to test how your actual application behaves when in use across all tiers.
If you are going to be load testing, definitely invest (time and or $) in a good tool.
