I am going to be doing my first tiered cake next week - 10 in on bottom and 6 in on top. I will have the 6 in on a cardboard cake board - do I also need dowels to support it?
Absolutely. You need a support system for every 4" of cake height. Dowel the base tier to support the top tier.
ditto. ALL tiered cakes need support.
Yes, you do need support for the top tier. Have people stacked at six inch on a ten and not have had a collapse? yes, it has happened. But for the simple effort of cutting some small dowels or even large straws the same length and pushing them into a cake...you will dramatically increase your chances of ending up with an upright and lovely decorated cake.
All one needs to support a 6"x4" round are 3 plastic drink straws (placed in a triangle fashion).