Deleting poor performing keywords is essential to maintaining an Adwords campaign. Each month you need to check your keywords and remove the poor performing keywords. It's like pruning dead branches to make a healthier plant.

Do you delete the keywords, or move them around to alternate ad groups and campaigns? It's quite possible to move keywords using the Adwords Editor. It depends on how you want to maintain your campaign history.

Adwords makes decisions on the quality score based partly on past performance. By only moving poor keywords to a back burner ad group you help Adwords maintain your account history. Adwords rewards account that have been active longer and have a thorough history.

Moving keywords will not affect your performance history. Google's optimization will move keywords from one ad group to another.

Deleting a keyword can affect the performance history, and damage your account to the extent of removing that history.

But, deleting keywords performs other useful functions. First, it prevents new impressions of your ad. If the keyword was not performing well, impressions without clicks will be dragging down the CTR of your entire campaign. Second, erasing certain history data may be necessary for a clean, fresh start. But be warned, not all history is deleted. The Adwords help section says...

“If you delete a keyword and then add it back to your account in any other format or any other location (placing it in another ad group, for instance), our system will still take the keyword’s past account-wide performance into consideration. A poor performer can affect an entire ad group and/or campaign, if it is used multiple times.”