Sometimes it doesn't matter -- sometimes it does. It will depend on the agency.
It is too bad that I've only caught this now (14, March'2015) however if it is helpful, I'd like to inform others: There are buses from Tambor direct to Montezum, although it does stop at Cabano to let of passengers and pick up other passengers for Montezuma. The buses from Tambor to Montezuma are coming from Paquera (the ferry terminal ). So far these are my recorded approximate times for the buses from Tambor to Montezuma (via Cobano ):
Anytime between 7.20am to 7.34am
Anytime between 8.55am to 9.05am
Anytime between 11.20am to 11.40am
Anytime between 1.20 pm to 1.50pm
There are buses at other times, however these are the times that I have recorded for my own interests. The bus schedules are never at a definite time, that is why I've listed them as between the times that I have recorded. It always pays to be a little earlier than trying to catch it at the exact time. If you miss it the the next bus will probably be 2 hours later. There are 3 bus stops in the town of Tambor. They are each about 1 to 2 minutes from the other. At the moment the fare from Tambor to Cabano is 500c, and the fare from Tambor to Montezuma is 1000c. If you are young with backpacks you can take this bus transportation with no problem. Older travellers with cabin luggage can do this pretty comfortably too. The locals even older or middle age heavy set women get on this bus with 2 or 3 shopping bags. The bus driver is very patient when one has to take a little extra time ascending or descending the bus.