Brandon,
If you can guarantee the quantities of gas and oil you add, you remember to close the petcock first and you can pick up your ped and shake it well, no problem.
In practice, ped tanks are so small, that if you try to mix in the tank, you wind up mixing odd quantities which makes it easy to screw up the ratio. Mixing exact amounts in a gas can avoids this problem and you can top off your ped's tank any time even if you are mathematically impaired.
Ron