Premiere: How to export at exact down-scaled resolution?
In general I would set my sequence settings to the desired delivery formats, and scale my source video to fit those. I wouldn't ever squash or stretch the video, as this looks very bad, and all iOS devices these days have square pixels (ie a pixel aspect ratio of 1:1 (or some multiple thereof if you're dealing with retina displays).
In general mp4 files (if that's what you are exporting to) prefer resolutions which are divisible by 16 ( I think - although someone on here may be able to correct me).
Your options are to rescale and zoom manually, shot by shot, to accept letterboxing, or to crop some of your content.  Rescaling and zooming to whatever is most relevant in each shot would be my preference.