I recently was involved in a bathroom renovation. Install new work boxes because walls were stripped. went back after it was sheetrocked. Found my 3 gang box was sort of slanted where the standard device srews were now too short. I normally just install longer screws. Any tips on how to avoid this?

If it was straight to begin with, it's probably just the product of a drywaller cutting the hole out not quite straight. You might just cut away the part holding your box at an angle and push it back into form.
As far as in the future, unless it's a really deep box, once you get above 2 gangs in plastic nail on boxes, I usually toe in a 2X4 behind the box and slip a couple drywall screws through the back for farside support if the box feels like its a bit "twisty". this also keeps your box at proper depth all the way across

Also there are sturdy boxes, and then there are some really weak ones. I prefer Union boxes for most things, and there is a really strong "strong" box at the big orange that stands up real well, although I do not like that style of built in box clamping method that the blue boxes employ.

Thanks for the info