dahlia_guy, that's a really good question. Oncidiinae are typically the worst for having brown spots. A haze of them, all tiny, will pepper the ends of the leaves, especially if the plant is given enough light to bloom well and profusely. The ends of the leaves will also turn slightly yellow. There's not a whole lot you can do about that, so most people have just learned to live with it, and to cut off the ends of the leaves into a V-shape when they become an eyesore. If you grow those under dimmer conditions, you might avoid the spotting somewhat, but the blooms won't be anywhere near as fantastic.
As far as leaf tips turning brown, that's usually caused by an over-application of fertilizer. If you haven't fertilized in a while and then do so suddenly, the salts in the fertilizer will literally rush to the ends of the leaves and burn them. So the key to avoiding that is to fertilize more often, at a weaker strength. (That's a tough one for a lot of folks to follow; we can't help but think that "just a little more" fertilizer will make the plant grow bigger / stronger / healthier, but that just isn't the case.)
Brown spots on leaves that indicate infection or disease are usually a lot larger, and they usually spread pretty quickly. The centers of those spots can get wet, mushy, and runny (indicating, more often than not, a bacterial infection), and, if left unchecked, such an infection spreading to the center of the plant can very easily kill it.
Nobody I know of "likes" to apply poison of any kind to their plants, so many home growers usually wait until there's a problem before doing any kind of spraying. We were the same way. But ever since we went commercial, we just can't afford to allow a problem to occur because invariably, putting a stop to a bacterial or fungal infection that's run rampant often involves cutting away the problem areas. It doesn't look so bad on Oncidiinae, but do that to a Phal and the plant will end up looking marred and disfigured. So we now use pesticides and fungicides on a regular basis as preventative measures. That's still not a guarantee that nothing will come up; we still get the occasional spot of fungus, (and I don't know any grower who doesn't) but we're pretty vigilant, so it happens infrequently. A regular spraying schedule (every three months or so) is something you might try if you're concerned.