I think it's OK to lean forward from the hips (not be strictly vertical), just keep the extension through your arms, and don't collapse or bend at the waist. Of course, don't trust me on that, ask your sensei or sempai.
I'd definitely agree with that - it's the curving of the back that breaks your strength over uke; a straight back and bent at the hips should make your sensei happy. It also helps me to angle my front toward uke's head a bit. Try it a few ways with a sempai watching to tell you how it looks! Good luck!