At least on earlier difficulties, there's no NEED for a build. Master traits can often point you in the right direction if you're unsure, but I could probably play a game on ITYTD, possibly even HNTR, without a single trait. (Sounds like an interesting challenge mode...if only it were something doable on harder difficulties.)
Man, you've never tried this? I once cleared H,NTR mort with SoG 3, Bru 3 and Ber. I didn't have a pistol or a melee weapon on me.
You're right. It has gone quite off topic. On a related subject, I was also curious about what kinds of items people mostly like to keep around?
I usually stock up on mostly health packs and at least two teleports if I can find them . I almost always use mods fairly quickly. At least 3 armors of the highest grade possible. I almost never use mods on armor.
I do try to touch upon this in my guide, but really I never keep a strict guideline in my mind. I always just stand on the stairs of a cleared level, tap i... "Hm. Probably too much ammo.", drop some shells and go find some other stuff to carry.
If you really need to be pointed in the right direction, ammo is usually the first thing you should think of. You should default to carrying a lot of ammo and only 5-6 consumables and a couple of extra weapons/armour, but there are a lot of situations where you can start dropping a lot of your ammo. For example, if you're trying to get ammochain, you should have barely any ammo when you're at level 6 and halfway to level 7, because once you get ammochain you're just going to drop it all anyway. Similarly, you should never have a lot of shotgun shells on levels 9-12, because that's a bit before when the halls of carnage spawns and that place is just filled to the brim with shotgun shells.
The other obvious one is how much ammo you should bring to level 25. Here's a tip: Only a bit more than required to kill the cyberdemon. As you get later in to the game, try to fill more slots with consumables and armour.
As for which consumables to take over which, I generally go Small Medpack < Phase Device < Large Medpack, with one envirosuit and 1-2 homing phases regardless of what other consumables I have.
As a final note, mod your armour. Power mods are super duper handy on a combat shotgun or chaingun, but aside from that you can also put them on red armour for a [6/6] arachno-fighting suit. Bulk can also be good if you're not finding a lot of armour, which will be handy in 0.9.9.3 because Kornel has decreased armour spawning and increased armour shard spawning.
Agility mods are less handy, but there are characters that wont have a lot of use for agility mods (Like a shotgunner, after they mod their rocket launcher and boots) and it's a lot better to pick up an item and immediately use it than just letting it sit there, let me tell you.