Mixing ingredients ahead of time
Typically, yes.  (it depends on the exact recipe).

There's actually a whole category of crafts / gifts for various 'mixes in a jar' where you mix up all of the dry goods, put it in a jar, decorate it, and label it w/ instructions on what else needs to be done.

update:

To clarify -- if the recipe calls for mixing all of the dry ingredients together before you start, there will be absolutely no problem.  If you specifically go out and search for "Mix in a Jar" recipes, you should have no problem.

The issue comes when the dry ingredients aren't added all at the same time -- sugar is often mixed with the wet ingredients ... it might be used for creaming the butter, or in a chiffon or angel food cake whipped with the egg whites.  In those cases, you may need to hold back an ingredient from the others, not pre-mix them.