Originally Posted by
findark
Yes, you can book Main with a Reserve certificate.
I have no issues doing this - in fact, I can only book the nonstop flights and it won't show me connections in First. When in doubt, call and wait patiently to get an agent who knows what they are doing.
I only ever call to let an agent help me find the best itinerary for my companion certs. But then, I tend to use them in scenarios in which I've got fairly flexible travel (they've all been personal travel with a family member, with flexible travel dates).