After the buses depart from the terminal, they go to Central American Boulevard, around the roundabout at Cemetery Road, and stop at a bus stop on the north-west corner of the intersection of CA Blvd & Cemetery Road. There is a taxi stop on the south side of Cemetery Road opposite the bus stop. It's a popular stop, so there should be several people waiting.
The buses continue west along Cemetery Road until it becomes the Western Highway. You could wait anywhere along that route, but the taxi fare will increase the further you get from the City, and you'll be standing in the sun (or rain).
I would suggest going to the bus terminal because there will be a better chance of getting a seat, and a better waiting area (you won't be standing in the sun).
Where are you coming from?
If you're flying to BC from San Pedro, do you know about the Tropic flight from SP to Belmopan? Is that any help?