Greyhound recently changed how they price tickets, so your experience in the past with pricing being based on time windows of x amount of days before departure is probably outdated.
Most tickets use a sophisticated yield-management model similar to airlines': prices change frequently based on things like demand and competitor pricing.
I work for bus a search portal, Busbud, where we partner with multiple bus companies including Greyhound and show them all in one place; we've noticed Greyhound matching prices from other companies like Megabus. So not only does the ticket agent not know when the price will change, but Greyhound isn't sure, they'll adjust the price to get as much money as they can while filling the bus as much as they can.
As part of this change Greyhound have also started charging much more for tickets bought at the station vs. bought online in most cases, so if you want the best price, book online.
You're still normally better off booking at least several weeks or days advance, but the bigger drivers of price going up will be how popular your travel day is (if you can find a day not many people want to travel you'll be rewarded with cheaper tickets) and if there aren't other bus companies competing on the route (if Greyhound has a monopoly they can charge a bit more, and probably need to, because the route is likely less busy or a competitor would step in).
This system also means Greyhound is now booking passengers onto specific buses, so you're no longer able to use a valid ticket for any bus, you need to show up to the time you booked. This has helped resolve issues around overcrowding on popular travel days: if you book a ticket, you're basically booking a specific seat.