I typically just use bs=32k as a happy middle ground. I found
this a while back and it sort of implied that anything bigger than 4k was about the same speed. Not sure if that's still true or if it depends on file type or file system type, but I've never had data loss with 32k and it's much faster than the default 512. I do many times use bs=512 when taking an image of things under 2GB (like camera SD cards, etc) when I'm doing data recovery. But that's probably more out of paranoia than anything.
