As far as I can tell, recluse spiders are the only spiders to have confirmed necrotic venom, but there's a lot of spiders and not all that much grant money for arachnology. I found the White tip spider you mentioned, but what is the yellow tip? Is it this glorious beast? If so, that's an orb weaver, and likelihood of necrosis or significant venom is pretty low.
No, it's not that glorious beast, nor all that glorious. I was told it was a "relative of a white tip", by a medical person when my brother was at the "oh god we might have to remove his leg" stage of the experience (They did not, in fact, remove the leg, yay!), and I don't know its Linaean name, so I'm at a bit of a loss. Golden orbs are pretty, but not considered dangerous over here. I love their webs, except when I walk into them at an ungodly hour of the morning.
Venomous animals in general, and spiders in particular, are major targets for popular mythology and urban legends, even among medical personnel. I'm from california, and there's a massive popular belief that brown recluses are present here, biting people, and causing horrible wounds. There are no brown recluses in california. About 6 have been found ever, pretty much all closely connected to recent moves or shipments from the midwest (recluses are reclusive. What happens when you take out all your furniture and the only dark place left is in your moving boxes?). Despite that, there have been thousands of diagnosed recluse bites, that all turned out to be things like staph aureus, early stage syphilis, lyme disease, etc. All things that could have been effectively treated if people weren't dumbasses. It's almost worse with snakes, but I won't get into that rant.
Yeah, it's a problem here, especially with the Australian folk tradition of playing up the venomous beasts we really do have (the only venomous mammal, for instance) and even inventing new ones (hoop snakes and drop bears only being the most famous. There's also everything from Nargun and MinMin onwards. We steal a lot from the local indigenous population, who also play this game). Add to that there are real things like Hexham Greys and random unfortunate incidents involving leeches of varied size, and folk stories get blown all out of proportion...
My brothers leg is an example which I was informed is quite common. He was moving boxes in a garage, got bitten, immediately sought medical advice, was told it was nothing and the bite was carefully sterilised "just in case". It wasn't nothing, so he went in the next day, with a much more large and angry looking wound, was again examined and sterilised, and was told he'd be fine, it was nothing. He went in again, day 3, to show off what was fast becoming necrotic tissue, and was told "oh no, you didn't give it medical aid when you were meant to...." and proceeded to watch everyone panic for the next few weeks while his thigh rotted. Not as bad as a dorf hospital I guess... I seem to remember him telling me that the spider he killed, and attributed to the injury, was less than an inch across, including legs, and was what Aussies on the East Coast call yellow tip spiders, due to a sort of yellowish splash on the end of their abdomen. Family opinion: He killed the spider and it cursed his flesh to die and rot. Don't kill spiders.
I agree completely with the dark place when moving boxes comment. Poor little bastards don't have a lot of options open to them, and they're so small, we don't usually see them untill too late.