I ran some test clips this afternoon. 720p60 with a 1/120 shutter definitely does not look right when converted to SD 30i - way too jittery. At 1/60 it was OK-ish.

I would stick with a 1/50th or 1/60th shutter. The reason for using a 180 degree (180 = half of 360 so half of the frame rate) shutter at slower frame rates such as 24/25 or 30P is to reduce image blur during camera moves or fast motion. If you do even the slightest camera move at 24P with no shutter (ie 1/24th) then the picture appears to go soft. A 1/48th (1/50, 1/60) shutter prevents the worst of this keeping the image sharp. If you look at stills photography the slowest recommended shutter speed for handheld is 1/50th as any slower than this and the pictures will be blurred. If your shooting at 50P/60P then there is no longer any need to use a 180 degree shutter as your shutter speed will already be fast enough to reduce image blur. Of course there may be artistic reasons for using faster or slower shutter speeds but this is why a 180 degree shutter is normal for lower frame rates.