Nope. I do it all the time.

The start/stop backup has to be issued on the master. It doesn't look like the standby gets the backup label (at least on 9.0, may have changed since). So you'd have to be reading from the master's data directory.

Alternately, you could stop the secondary and pull from there. But that interrupts the replication, and then the secondary would have to catch up, which might be hard depending on your level of usage.

feel free to email or chat my un on freenode.