I think it probably really depends on the horse and the type of work you're doing with said horse.
I think I've also read/heard that there's a bit-metal component too - some metals are going to make a horse salivate more than others.
For instance, the only bits I've ever seen Lacey get "slobbery" about are low ported. She goes just fine in a mullen mouth, hates a broken bit, but never gets slobbery.
Put her in her sweet iron low ported curb and she gets somewhat slobbery. Put her in her stainless steel low ported hanging-fulmer snaffle (used to be a "rugby pelham" removed the shanks, voila) and you would think that she has suddenly developed a fountain for a mouth.
I initially thought she would just never slobber on anything but it turns out that it's all about the bit. Unsurprisingly, she's the most relaxed and comfortable in the hanging-fulmer.
Obviously you don't want too much slobber (too much can be a sign of nervousness) but some is good! :)