Filler is great for hiding the pits and tiny imperfections in the metal, and if used sparingly, should be alright. I like to use high-build primers for that job, but you must be sure to let them cure for a while before applying the final paint. (the solvents must have time to evaporate and the primer will shrink some -- potentially resulting in defects showing up in the top coat later down the road.)
When I cut and sectioned the grille for my son's Commando 7, I tried to do as much as I could to limit the use of filler of any sort. When you're talking about a 10 year old kid handling a hood, plus actually driving the tractor EVERYWHERE, you become less fussy in a hurry. I did use a
very light skim coat of plastic filler to cover the rougher areas, but most everything else was handled by a high-solids primer/surfacer over a base coat of etching primer. (I actually prefer Dupli-Color "rattle can" etching primer that I buy at NAPA)
He beat the
snot out of that tractor for 3 years and I never saw signs of cracking or lifting of the filler. All I can add is to spend as much time as you can with the metal. Fillers -- especially plastic -- are not made to fill dents more than 1/8" deep, but I have seen it as thick as 1 inch in some "short-cut" repairs. As soon as moisture gets behind plastic filler, you have problems. Use a waterproof filler such as Dura-Glas first, to help seal the surface. (not normally needed if it's solid metal requiring dent/pit removal only.)
Buy or build various shaped dollies and body hammers if you can, but you can also use what ever you have laying around. I have a 120 lb. anvil that helps me out a lot in the fabrication of metal, and I have a custom made "spoon" dolly that I use to reshape the round hood grille bars too. A heavy sandbag is also used as a "backer" for certain hammer work instead of a solid surface. A lot of times I actually work the grille bars and other similar areas while holding them against my leg!
A metal file is nice to have around too (for knocking down high spots) as is a tool we used to call a "slap hammer", which is nothing more than a large rough-cut hand file, heated and offset bent in the center with a piece of strap steel welded to the back side for rigidity. It's used mostly for cold shrinking/leveling of metal, and the tiny nicks it produces will help the filler bond to the metal. It does require a solid dolly on the back side of the metal.
Heat shrinking is needed quite a bit on some of the more damaged sheetmetal and isn't really that tough to master. Use a small welding tip on an O/A torch and heat a small spot to a dark red-orange color. While the area is still hot, hit it with cold water. Repeat the process as needed, and keep moving around the damaged area. (Supposedly the molecules in the metal will realign and condense, tightening the metal.) All I know is that my shop teacher showed me how to do it, and it works.

It can even help take the "oil can" effect out of the top of a hood if you're careful with the heat.
Well, you guys are lucky....... it's supper time here and I'm gonna quit typing!
That was more than enough to bore you with for a while anyhow.