Where to put biscuits on a chess board.

[Slipknot]

you can use biscuits for the perimeter if you want to, but even better and easier assembly might be a full spline if you are mitering the perimeter.

One other note, are you fastening the top to the plywood with screws through slotted holes from beneath? that will help with expansion and contraction between the solid and the plywood since the solid wood will want to move and the plywood is stable.