The precise elevation depends on the source. According to SummitPost
SummitPost page of Main Whitney Trail the elevation is 9,420 ft. On this, the WPS website, the elevation is 9,950 ft - but it's listed as the
fork in the trail that leads
down to Lone Pine Lake.
There are other elevations that are subject to debate - the most prominent being Trail Crest, which is listed on the sign as 13,600. The SummitPost site lists it at 13,777.
I do find it interesting that, given the vagaries of the reported elevation of LPL and Trail Crest, the plaque at the summit is precise to three decimal places.
I did have a thought. When I went up, I had considered taking my Garmin Nuvi with me. It does read elevations to the foot. I left it behind, since I didn't need the extra weight, nor did I figure on getting lost when the weather was so nice.