I recently bought a Pfaff 360. I've used sewing machines before, but this is the first I've actually owned and I don't recall this being an issue I've noticed. I believe I have the tension settings correct (based on the "dangling bobbin" test, and adjusting the upper dial), but it looks like straight stitches are slightly slanted.
The needle entry points look OK, so I don't think this is an issue with one of the zig-zag cams being engaged by mistake. For example here I've just run some fabric through to puncture it:
my accuracy in sewing a straight line/fabric stretch aside this looks like random jitter to me.
I've also read that this can be caused by torsion in the thread, which is just some Gütermann polyester. But I wondered if I have some remaining tension issues. I wouldn't say the stitches are obviously loose and with different top thread/bobbin colours they're well separated.
Here's a GIF - it does look like the needle is coming down slightly at an angle, but it's hard to really tell if this is an optical illusion or not (I'll get a video with less shaky cam).