This question could provoke a bit of argument, there are a lot of people out there still avidly supporting IE6, and others like me who wish that older browsers would just magically disappear so we don't have to consider them.
Anyway, here's an answer for your question:
you need to define "web app". If you mean a web application that is used by businesses in a LOB type of way, then you should be able to dictate the minimum requirements. For the web app i am currently working on, we have a minimum of IE8 or FFox 3.6. In only one case have we had a client who could not upgrade from IE6 because they were still using a critical legacy app that was built for IE6, so they use FFox for our app instead.
If your web app is going to be exposed to the wider world, and any random John Q. Public can use it, then you certainly still need to consider the numbers for each browser, and then make a decision of whether the work involved in supporting that browser is worth it for the number of people using it.
