> I only want to create an index on a few important fields that will be regularly ordered by web visitors.

Who am I do deny you to do stupid things?..

> And I really do want to allow both ascending and descending orders. This being the case, should I create two indexes for my example timestamp field in both ascend and descend order?

Once again: you don't.
Indexes are useful for data filtering. For ordering they are less useful.

SY, SD.