No. "Spawn" in multiplayer is a single coordinate, but new players (and unless modified by a plugin, all respawns) can happen in any 10x10 area around that block, centered on the NW corner of the block (in Single Player, the area is 20x20). The spawn will always occur on the highest block of the particular column that the spawn algorithm decided to put them in. If I made one block in that 10x10 at y=255, then 1 out of every 100 new players would be unlucky enough to spawn on that block. Height or transparency has absolutely nothing to do with it.