I disagree with the people saying that the GPS will be accurate enough to be certain that you're not trespassing. The GPS chips in cell phones can't be trusted to be more accurate that 50 to 100 feet.
That said, personally I would just go with the phone if there were no signs posted or other indications that the landowner is sensitive. However, if you get there and it's posted, you'll need to go to with a more reliable system. The best thing would be some other landmark to confirm the GPS on your phone. I'm going to be really certain of things before I ignore no trespassing signs.