>>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.
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>I removed that from the web.config so I want to make sure the latest framework would be used.
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>>BTW, the settings in web.config are automatically updated if you change the 'Target Framework' in the 'Project Properties' page...
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>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.

No. I was assuming that you publish the development web.config (or at least a web.release.config) when deploying. Changing the target framework on the deployed app could be risky if you've compiled your dlls for a different version....