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Trevor.. A few thoughts..
1 - if your field of view is not flat, and your target is not in exactly the same position in the field of view (both location and camera rotation) any stacking/registering program will have troubles... Even registar. a curved field of view will always result in warping of the star positions, this will result in odd looking stars when stacked..
2 - Flats are critical..
What I do (and I've done 4~5 night imaging runs from different locations etc, sometimes weeks/months apart.) is firstly, get your camera orientation as close to the same as possible.. a degree or two wont matter, but definitely, the closer to perfect you can get the better... Open up one of the previous nights images in your capture program (ie. Maxim, Nebulosity etc) and then take a new exposure, switch between the new exposure and the previous nights exposure to see any difference in composure and or camera rotation... from there, figure out what needs to be done to get them as close to the same as possible. Once you've got it all sorted, take your images...
That should ensure that your night to night exposures line up well enough that you only need to crop out a small section of the field of view.
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