What browsers do your customers use, and which browsers should you be testing? One way to answer this question is to consult the several services that track browser usage, and design your testing strategy to match (for example, here, here, or here). These sites are free and are definitely better than pure guessing. These sites do have some good use, you can see trends over time.
However, a better way is to use analytics within your application, and find out the browser usage for your customer base. The actual usage by your customers will likely vary from the published sites. For example, on my project we currently have 1.8% of our users connecting with IE6. While NetMarketShare shows nearly 17% world wide usage. We have a very different test strategy for a browser with 1.8% usage than one with 17%.
Analytics doesn’t have to be expensive. It may, in fact, be free. Google Analytics is free to use, and provides a lot of data about your customers. Another free source of browser analytics is parsing the log files from your web server, aggregating the User Agent Strings.
Test for your customer, not a generic customer.