My personal view on this would be that they have proved themselves to be a reliable tennant, have hit on some hard times and i would be perfectly agreeable to go with the rent allowance. Its not so much of a risk as taking it from day one because you know have a relationship with the person and can make that call much more easily. And i would be fine woth it as long as they were up front with me from the start.
The reason I dont accept rent allowance up front anymore is that its too easy for the person to just mess you around and do untold damage and walk away without penalty. I prefer to negate this by getting tennants with a job and references and not someone who is depending on a third party to give them their rent and could say at any stage "Sorry, they wont give me rent allowance, tough." It has happened.
Having said that I sometimes do allow people on rent allowance upfront, but its purely down to if i like them, but i never advertise that i accept it.