De acordo com uma fonte :
As the ticket policy suggests, Greyhound does not assign you a seat when you buy a ticket. If you show up at 8 AM for your 8 AM bus, the bus may be full. If you show up at 7:30 AM and the bus line already has 55 people in it, you may not get to ride that bus. If you're riding for your first time, you will get a good seat on your bus if you arrive an hour ahead of time, and will probably catch it (unless you need to check bags) if you arrive fifteen minutes ahead of time.
For the expert, how early you show up can be calculated by four variables: (1) Are you checking bags? (2) Is your bus "originating?" (3) Is your departure station popular? (4) Is the route popular?
If you're checking bags, you will have to wait in line. Depending on station , you may have to wait in the general ticketing line. At peak times, there can be 30 people in the general ticketing line--all of which have urgent needs. You don't have a choice about waiting, so budget 15-30 minutes.
An "originating" bus means that your bus will be empty when boarding begins. If your bus is originating, you can show up later, because more seats will be available.
If your departure station is popular (i.e. a large city), more people will be in line, so budget another 15 minutes. If the route is popular (and most are, especially on the weekends and at night), you may wish to budget another 15-30 minutes.
Your best situation is an originating bus, leaving from small town X, at 10 AM. Your worst situation is a North Hollywood bus originating in Los Angeles, CA going to San Francisco at 10 PM on a Friday night.
Então, considerando tudo isso, eu tentaria chegar o mais cedo possível. Porque afinal de contas, este é Ação de Graças , e você já será grato por ter um assento;)