I am not sure when a straight flute is to be used other than maybe a finishing pass. Dremel’s have horrible runout, anything harder than wood is going to be tough.
Upcut or down cut and best to make sure the end is meant for plunge cutting. I am pretty sure most straight flute can not plunge cut.
Here is a quick description, http://blog.cnccookbook.com/2012/04/06/cnc-router-cutters-types-and-how-to-use-them/