I actually prefer bits with a mild port, it gives the horse some tongue clearance and helps minimize the urge to gape his mouth (which is fairly common with bits that press onto his tongue). If the horse has been taught to carry the bit himself, then it isn't that big of a deal if there is no port because there is hardly ever any actual pressure on his mouth, but for horses who just pack the bit and don't really try to feel it, they'll often gape when you pick up pressure. Depending on the horse's preference, a port can make the difference between a horse that looks willing and happy
And one that looks like you're yanking his face off, even if you aren't
I am a fan of the Sweetwater type ports because there is enough arch there for good tongue relief, but they generally aren't big enough to interfere with the palate unless the horse has a very
BTW, this is a sweetwater port
While that is the bit that I've used for years on my horses after they graduate up from a snaffle, I recently got one of these ( AT Low Port Loose Cheek Low Port Western Bit 5in - Horse.com
) and I am really liking it. It offers a lot more feel and mobility than the solid port without collapsing on the horse's face the way a broken bit will whenever you pick up the reins. I also refuse to use anything with stationary shanks. Swivel shanks are a must on all my curbs.
Anyway, as far as when to bump them up. I expect them to be soft, supple, and responsive. They should have a pretty dang good idea about neck reining and responding consistently to light cues on their neck. It doesn't have to be perfect because, let's face it, even older broke horses will need a bit of one-rein correction occasionally (that's where the swivel shanks really come into play). As you said, a great deal of it also depends on the individual horse. Of the horses I've worked with, it's ranged from around 10 rides to around 60 rides before they are in a curb bit. It depends on their mentality, how eager they are, how willing to learn, and how they feel in my hands. There has been more than one occasion when I put them in a curb and immediately though "Eh, this just doesn't feel right", so back to a snaffle they go for a while longer.