Same as I have for years now: I freehand them, and then polish them up after scanning.
Although, recently my eye for perspective has gotten good enough for me to construct some backgrounds entirely with my pen mouse, as long as they're relatively simple and uncluttered...like these stair shots, which turned out surprisingly well. I don't think I would've been able to do this three years ago:
Lately I've been trying to take my own advice and focus on constructing backgrounds to look cute and fill up the negative space in a fun way, rather than to 100% accurately show what would be around the characters from a given angle. Like, y'know, stylization. ^^;
And I think one of the reasons so many artists dislike drawing backgrounds is because that's so rarely encouraged...
Like, imagine if whenever you looked at a tutorial or opened up a comic, all the people were drawn photorealistically, with clean vector lines. And the most common advice given to learn to draw people was to just 'trace over photos of human beings'. You can imagine how many artists would be frustrated with that, and probably grow up thinking they just hated drawing people...that's basically what's going on, only with backgrounds.
And it's sad, because there are so many more things you can do...like, I have nothing against tracing/painting over 3D models, but I think the popularity of that technique has made a lot of people think their only options are to either use it or spend many miserable hours trying to replicate it. :[
Unfortunately, I myself am not skilled enough to give advice about alternatives...I think the main reason I still don't like doing backgrounds is because I'm in an "experimental" stage with them, still trying to figure out what works and doesn't work for me at this point in time. But once I get out of it, I'd like to try to do some tutorials. ^^ Color theory and shape language can work just as hard for you as a perspective grid, if you take the time to learn how to use them.
I would say "as rarely as I can get away with" but that's actually not true. ^^; I still tend to think like an animator, rather than a comic artist, so backgrounds are usually an important part of most of the shots I come up with. If anything, I often have to revise my paneling to include fewer background elements, because I know I won't have the energy to draw them all.
I think this fancomic is a good example of how I ideally 'ration out' background work:
When there are fully-drawn backgrounds, they can get a little wild (like that bird's eye-ish shot in the third to last panel...to this day I don't know why I decided to do that to myself) but most of them are minimal, and about half the panels have either basic shapes or no BG at all.* That's a very comfortable ratio for me~
*BTW, it's a LOT easier to get away with that stuff in monochrome, and if doing backgrounds is absolute hell for any of you out there, I implore you to give a B&W comic a try, at least once. Screw people who only read full-color comics; your enjoyment and comfort is more important than any 'potential fan' with such a shallow mindset...