Strive for Validity

Remember that in Solid Modeling, your primary goal should be to produce a clean and valid solid. Try to avoid operations that may cause unpredictable results. Do not depend on the application to alert you as soon as a problem occurs. It is always better to undo and redo operations than it is to fix the results of a bad operation. Always assume that a problem area that is not undone now will cause problems later.

Source
Don LaCourse, Principal Partner, eDocHelp