I've recently been getting a lot lazier in my prep work on cast parts - I'm using more and more of the phosphoric acid and painting right over it.
I used to sandblast everything extensively, but now more often just wire brush instead (in an angle grinder, not a drill!)
Just enough to get rid of what rust/old paint I can with a reasonable effort, but I no longer kill myself trying to get every bit of it. Just hit what's left with acid, it all turns black.
Sometimes I'll repeat the wirebrush/acid step if it's in real rough shape.
Then I'll quickly wire brush again right before painting just to get it all scuffed up and get rid of any built up excess acid residue. The acid has the added benefit of buying you time. You can brush and spray the acid (I use a spray bottle) and let it sit several days without worrying about more rust.
I usually use epoxy primer.
I can't tell you what the long term results are because I've only started getting lazy with this process fairly recently - but I can tell you it's lasted me a solid year without the slightest hint of trouble...
Time will tell.
I guess it boils down to how much you care about whatever it is you're painting. If you're spraying $500 a gallon paint, then I think it's worth the effort to get all the rust off.
But for most stuff, I figure if I have to repaint in a few years, so be it. Anything I bother painting stays well maintained afterwards, so a future paint job won't be that big of a deal.