If you have to, you can change later- though from experience, there's little need to with .NET. I work for RecordSetter (http://www.recordsetter.com) and we use ASP.NET MVC- back in the early days of the company, the CTO knew .NET very well, so he just used his existing knowledge. C# is a great language, Visual Studio is a great IDE, MVC is a great framework. And so on and so on. We aren't really suffering any ill effects as a result of our choice, and as others have noted, BizSpark membership is actually a great bonus.
If anyone out there wants to play around with C# for the web but feels like the ASP.NET stack is too heavy (it can be), take a look at Nancy.FX (http://www.nancyfx.org/). It's super lightweight and allows you to keep most of .NET out of your way.