>Yes. If you target, say, 4.0 then any attempt to use newer functionality in 4.5.1 will not be available. In most cases that mean that the app won't compile. In a few obscure cases you may just get different behaviour from that which was expected.

I removed that from the web.config so I want to make sure the latest framework would be used.

>BTW, the settings in web.config are automatically updated if you change the 'Target Framework' in the 'Project Properties' page...

If this is in reference to my local Web server where the PC used for develpment is I assume. I do not think an upload of a DLL in the Bin directory of a production server, for example, would force a change in the web.config.