Not that big a deal I guess but I'm rebuilding a crumby old mountain bike for a neighbor to commute on and I went through every hub in my junk pile trying to find a straight axle. Not one.
No biggy, the LBS has them for a couple of buck, brand new. I get home, put it in and voila! It's bent, right out of the box

I was beginning to think so last weekend and found one donor one from another wheel that wasn't bent. The one I took out was bent on both ends about where the nut tightens up to the frame.

No, some are snapped in two

My '78 tandem for some reason had thin 10x1 axles, they were bent beyond recognition. But the thick axles on the many gaspipe commuters I've rebuilt have always been straight.

A cup-and-cone hub is usually self-aligning to some degree and can handle slightly bent axles. Won't be optimal, of course, but they can handle it.

Good Lloyd I think you're right. It might be related to how almost all used MTBs are purple.

purple! yea- I got at least 5 purple ones behind the shed!

How are you determining that it has a significant bend? I'd think it would have been most obvious right when you picked it up at the LBS or before you put it into the hub.

The comment that you only noticed the bend after you installed it makes me wonder if you're seeing the usual (and normal) slight wobble of the freewheel which is unrelated to the straightness of the axle. It's been awhile since I had a bike with a solid axle, but I don't remember any problem getting a straight replacement when the one on my old Schwinn Varsity snapped.

Well, my eyes aren't good enough to see some of the bends while holding the thing in my hand but when I roll it across my kitchen counter top you can definitely see that it has an eccentric roll (I'm pretty sure it's not the counter top. I can also see it when it's in the hub and I spin it. The freewheel isn't on yet but you can see the deflection, in this case just less than a millimeter.
I had an axle that was bent mostly on one side once and I was trying to true the wheel and I'd get it perfect and the unwittingly rotate the axle a little while installing it in the fork and the rim would hit the brake shoe. I'd true it up again and install it and it would happen all over again. When I finally figured it out I tried turning the axle all the way round with the wheel in the dropouts and the rim would drift all the way from one brake pad to the other.
It's a lesson I have to relearn every time I rebuild a cheap bike. I'm building fewer and fewer cheap bikes but I do have friends who can't afford anything better.