While on another vehicle make forum I read a thread concerning a vehicle needing it's intake valves cleaned due to carbon build up around 45,000 miles. It was opinionated that all direct injection motors are subject to quicker carbon build up. Since Mazda has used direct injection in it's Skyactiv motors for several years I was wondering if anyone has experienced this? Thank you.

 

Can't comment on carbon build up for direct injection vrs indirect. Carbon is always there, especially if you drive short trips. EGR doesn't help the situation.
This is why I always put this stuff in every other tank full, a bit:

Red Line Synthetic Oil - Gasoline Fuel Additives - SI-1 Complete Fuel System Cleaner

It might not clean anything in intake manifold area though, as this is where the EGR crap goes by as well as fresh air. So i can see intake manifold, intake valves getting carboned up.