This doesn’t really happen that often anymore since most dcs that would have this effect on a game are done as redos. That said...
I personally let them farm as long as it’s not Nasus or Veigar, where doing so accelerates their scaling absurdly quickly. They’re already behind, and frankly they’ve already “lost” lane at this point. If they decide to start fighting me for whatever reason (which has happened before), all bets are off. I’m booty blasting you into next century.
I would love for someone to return this courtesy to me, so I can only afford this to my opponents out of principal. However, I do not except this treatment. In fact, when this has happened to me, I tend to not even lane. I’ll soak exp under tower and farm what I can, but once the minions leave tower, I leave.
In short, I sit somewhere in between. I believe you should try to let people farm, and as such I do it. But I don’t expect it and definitely see the opposing argument as to this being a competitive mode and such shortcuts should never be offered. As such, I incorporate my own philosophies without expecting them in return.