I had lengthy emails with the Kentico support team and couldn't find a way to make the exact phrase match work, nor searching by tag IDs. I have, however, come up with a work-around.
I've replaced the spaces in the phrase it with "+" signs to require each word.
i.e. +documenttags:(Cash+Flow)
In order to search for multiple terms, I use the following:
+documenttags:((Cash+Flow) || (Petty+Cash))