Solution files are not deployed or visible to webserver, so you don't need to worry about case in their names.
As far as aspx pages are concerned, most search engines are case sensitive when they index pages, so MyPage.aspx
and mypage.aspx
are not the same page as far as many search engines are concerned.
If you create pages that have upper case characters you could introduce situations where you will see search results for your page split between the lowercase and mixed case versions. This can adversely affect your the page rank, causing it to show up lower in search results than it would otherwise.
More importantly, you want to make sure that internally within you code you always build links that are consistent. If you're going to use mixed case - make sure you always use the same case. Just realize that you can't control what other websites choose to do - and many of them convert urls to lowercase. Also, most human users (when not copy/pasting) also tend to use lowercase for URLs.