First of all, there's no "o" in tung oil. Here's some background: Tung oil - from Wikipedia, the free encyclopedia
If you're using pure oil then thinning the first couple of coats will help it penetrate the wood fibers and allow you to build a finish quicker with the undiluted oil afterwards. If you're using a "tung oil finish" then it's probably already thinned, don't think it would help any to thin it further.
As far as fillers go, whether to use them or not depends on the wood, how you want the final piece to look and whether or not you want to do any "enhancements" to the appearance of the wood.
Fillers are most commonly used on open-pored woods like oak, mahogany, ash and, to a lesser extent, walnut when a "glass-smooth" finish is the desired result. The filler literally fills the open pores of the wood and when sanded leaves a flat surface for the finish to go over. They're not absolutely necessary(as you've discovered) but they eliminate the need for multiple coats of finish that have to be sanded back between coats to achieve a truly level surface. They're not needed on woods like maple and cherry which have a small, tight pore structure.
They can also be used in contrasting or artificial colors to accentuate the grain structure of the piece.
As you're finding out, finishing is a whole 'nother aspect of the craft. For some, it's a chore. For others, it's an opportunity to enhance the beauty of the wood and express an artistic vision. Personally, I feel that when I've glued up the piece and finish-sanded it I'm about halfway done. The other half is the finishing.... And the older I get the more I understand that this step is just as important and deserves the same time and care that I put into the first part.
Recommended reading: Understanding Wood Finishing
P.S. Spell check is a function of the web browser you're using, hunt around in "tools-options" or "edit-preferences" and you'll find it.