GPS needs three satellites to get a fix, most modern units are 12 channel, therefor capable of receiving data from plenty of the satellites up there i.e, the ones nearer the horizon. Yes the accuracy in altitude is diminished due to the geometry of the configuration, but average change in vertical speed is low, especially when considered as a factor of forward speed. given this ratio, and the perpendicular relationship between horizontal and vertical planes, I'd say the effect is largely negilgable, under normal circumstances (rolling road excluded:)).
Certainly within the scope of 'is GPS accurate?' the answer is yes, very. Ultimately the errors come down to the quality of the reciever. There was/is an argument for DGPS, but since the US turned SA off, you really can't go far wrong.