If you spend a lot of time removing the formatting from text that you copy into Office documents (say, text from emails or web pages into Microsoft Word), you should adjust your default paste options.
The quickest way to do this in Office 2007 or Office 2010 is just to paste some text into your document and look for the smart tag icon:
Click the smart tag to open your Paste Options (note that smart tags in Office 2007 do not use the icons):
You could click the Keep Text Only icon (on the far right) but if you want to adjust your defaults, select Set Default Paste to open the Office application's options:
Set the defaults you want (I chose Keep Text Only for all) and click OK. Repeat for all of the Office applications you use regularly (Excel, PowerPoint and Word).
If you decide that you want to maintain the original formatting for a specific paste, you can easily override it on a paste-by-paste basis.