I just got the Twin Peaks Pilot laserdisc and the US Box Set of Season 1. At what point during the pilot do I need to turn it off so I don't see the added footage which gives away the ending?

I think the different ending starts around the point Laura Palmer's mother has her vision and sits bolt upright, waking from her dream (I think that's the end of the US pilot). Up to that point the EU pilot is the same as the US one. If you don't want to spoil anything then just watch the US one instead. The EU pilot is interesting viewing but definitely something to be only watched after you've seen the whole series.