It's not dangerous.
The methadone is a vasodilator, which increases body capacitance.
This lowers blood pressure a tad.
Under certain circumstances you could become slightly dehydrated, or not have as much water available in the tissues as would be optimal.
Try to keep well hydrated. Especially in the summer. Diabetics tend to use of water in the system a little more than normal people.
Other than that there should be no big problem.
Are you blood sugars well controlled? That would be the key as to whether methadone has any effect on blood sugar-testing. You could also talk to your doctor or to the doctor at the methadone clinic who might actually know more about any possible effect for diabetics.