I was wondering the same thing!! So I asked Google, and he gave me this: http://forum.wordreference.com/showthread.php?t=392563
I hope this is correct ;P and helpful
Some liaisons are mandatory, some are forbidden, and a great many are optional. Of the latter, some are frequently heard in conversation, and some are very rarely heard.
Liaison after a verb (other than forms of être) is optional and rarely heard.