Upscaling to 1080p will only be done once. If both your Blu-ray and your amp are set to upscale to 1080p, your Blu-ray player will do it as it's the first link in the chain - your amp and TV will not do anything. If you set the Blu-ray player to not upscale when playing DVDs, but your amp is set to upscale to 1080p, then it will then do this. Again, your TV will not do anything in this instance, just display the signal it receives. Finally, if neither the Blu-ray nor the amp are set to upscale, then the TV will upscale the image to fit its native resolution - assuming it's a Full HD set, this will be 1080p.
As Lee H noted, there's no rule as to which is best, it depends on your equipment. The best is to try switching off the Blu-ray upscaling (by setting the output to Original or 576p) and watch a DVD and see how the amp performs. Then switch off the amp's upscaling and see how it looks when the TV is doing it. Compare each and go with the one you prefer. Just remember, if you have to manually set your Blu-ray to 576p, you'll need to change it back to 1080p when watching Blu-rays!