I find that the night vision mode of SkySafari, combined with a modest overall brightness setting, works well and preserves my night vision. Different generations of iPhones may have different amount of backlight bleed, but on the iPhones 4 to 5 (retina displays) this isn't a problem.
By far the biggest light polluter around my setup is the power LED of the HEQ5 Pro mount
Cheers
Steffen.