I think you've got a bit of a conceptual mis-fire here.
"Compression" (you're actually referring to minification) of static assets is a great practice. It saves you valuable bandwidth, as well as increasing the snappiness of page loads for your end users, which is always a good thing.
When you ask for "compression" of .aspx files in this context you misunderstand that the client is not delivered a .aspx file, they are delivered a long string of html, so minifying the .aspx could be completely pointless if you then go and bind a large amount of un-minified html to a control. You'd have to run this process after each page load in order to reap any benefit. However, I think you are asking about this mainly because you have not made the distinction between minification and compression.
Minification of dynamic assets is, generally, a bad thing. You're going to expend more CPU time running the minifier over your generated html page, and you're not going to see much benefit in terms of bandwidth unless your pages are very heavyweight.
What you are most likely to be looking for is actual compression, such as gzip or deflate, which will compress the output of your .aspx. This is actually an IIS setting. This will, again, increase CPU usage, but will (theoretically) provide more of a benefit than simply removing whitespace. Obviously this also applies to static assets, which can be massively reduced in size by the combination of minification and compression.