If I understood and recall correctly, you cannot buy a room on a rail pass until six months before travel. This used to be a big disadvantage because the rooms opened (11 months out) at low bucket, and the low bucket rooms could be all gone by the six month point. Now rooms are first available at an intermediate bucket, but you are still gambling with the price of the room.
Since I always expect to have another Amtrak trip somewhere over the horizon, I do not mind collecting vouchers by rebooking a room when the price drops. When I plan a trip, often almost a year in advance, I reserve the rooms (and pay for them, of course) and then set an Amsnag fare alert for my travel date and itinerary. If the price drops, I rebook and get a voucher, which I then use the next time I pay for a trip. And on it goes.
As noted, buying a room gets one the lowest bucket Value fare, but that means nothing if one is traveling on a rail pass. If you want to travel in a sleeper, I would suggest just making the reservation normally and not use a rail pass. If you expect to have no further Amtrak reservations in a year (when the voucher would expire), you could take a cash refund with a 10% penalty (if your initial departure date is at least two weeks away, again if I recall correctly).