I generally found it useful to pay.
However, here is a warning:
Word tracker lists some copyrighted words in their lists, without identifying them.
I ended up inadvertently using one of those (not in the casino field) and ended up being sued for a million bucks.
This is not some silly suit, it is well founded and filed and all and costing plenty to deal with.
So, be careful when you use terms from wordtracker. If in doubt, check for copy right!!!!