I usually male 4" tiers. But now have to make a 6" tier and wanted to get opinions.
Can I stack a 6" tier with no cake boards in between.

I know that serving a 6" tall piece of cake is not very appealing, but wanted opinions.

You can do 6" tiers without adding boards. I have done this several times. If you are covering your cake in fondant, they're pretty stable but if you're decorating in buttercream, they tend to be a little "wobbly".
That being said, I drove this buttercream iced cake...

http://cakecentral.com/gallery/2266242/valentines-wedding ...

through a blizzard last February in the Appalachian Mountain region of southwest Virginia without adding extra boards to my 6" tier and it arrived in perfect condition.

HTH.

Thank you so much. I am using fondant.
Did you put any support in the cake. I will have a center dowel because it's a dragon with a head. Picture attached or where I am so far, and another of the cake I need to replicate

Oh that will be more than fine. You are carving it and you have the central PVC dowel. Once it is covered in fondant that won't go anywhere unless it gets really warm.

Thank you