Hey @Bundaboy
good question mate and as I have seen that done in photos of tables, I didn't with my one. What I did was once I made the frame up from just using timber, I then ran another strip of timber aboot 60mm wide along each side of the frame and one side hanging about 30mm into the middle so each plank of Pallet sat on that little groove nicely and I then screwed from underneath through the strip into the top angled pallets.
i hope this answered your question
if you were use ply underneath, I'd say every plank of pallet would have to be the exact same thickness
