If the cake is not moving after assembly do you need to dowel and use a cake board seperating a two tiered cake. The sizes are 8" and 6" rounds?
I am horrible at getting my dowels the same hieght 
. The cake is for a birthday party for my niece at my house, so I won't have to transport it.
I'm sure you will get many differing opinions about this one.
I took a wedding cake course in May where we made 5 different cakes all 6" on top of a 10". The owner/instructor never used boards or dowels with these cakes. I was very hesitant but they actually made it home every time which was a 45 minute drive home in a Jeep Wrangler.
I don't think I would worry about the doweling since your not transporting them and the layers aren't that big/heavy (if your not using fondant). I have also started using the 1/4" foam board between layers (covered with Reynolds white freezer paper) instead of the card board cake circles.
Good luck with your cake and please post your picture! 
Here's a little tip to get your dowels the same length.
measure the first one, cut it and then use it as a guide to cut the remaining ones, this will insure that they are all the same height, and the layer that sits on top of them will be level, don't panic if some seem to stick out of the top of the cake more that's just where your icing is not as thick. As you practice you will be able to get your icing to the same thickness across the top of your cake, I measure the middle one first.
Hope this helps!
Jibbies
BTW, I would dowel even an 8 and 6 inch you never know what can happen.
I would still use a cake board if for no other reason that to make it easier to cut. If you don't, then it will be harder to stop the cut at the bottom of the 6 inch, and you're likely to end up cutting into the 8 inch.
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