The best practice is to always book DoubleClick for Advertisers (DFA) tags using the 'DoubleClick tag' creative type. Using this creative type will save you from having to add any additional macros for click tracking, and it will also keep the discrepancy of clicks between DFA and DFP under 2%.
It’s possible to traffic a DFA tag as a custom creative to allow for additional formatting of the creative, such as the addition of a third-party tracking pixel. In these rare cases, you must add both the DFP click and CacheBuster macros to the DFA tags in order to track clicks properly.
Here's an example of properly-formatted DFA tag (including the necessary macros) for the 'Code snippet' box in the creative's settings:
<SCRIPT language='JavaScript1.1'
SRC="http://ad.doubleclick.net/
<NOSCRIPT>
<A HREF="%%CLICK_URL_UNESC%%
<IMG SRC="http://ad.doubleclick.net/
BORDER=0 WIDTH=728 HEIGHT=90 ALT="Click Here"></A></NOSCRIPT>
<NOSCRIPT>
<A HREF="%%CLICK_URL_UNESC%%
<IMG SRC="http://ad.doubleclick.net/
BORDER=0 WIDTH=728 HEIGHT=90 ALT="Click Here"></A></NOSCRIPT>
When a DFA tag is trafficked as a custom creative, DFP will treat the DFA tag as a third-party tag and therefore will not report clicks without the DFP click macro. Additionally, you should expect to see a larger discrepancy in delivery numbers, as you would with any third-party ad server.