I am not positive which wing Sherman accompanied at the time the right wing passed through Madison, but I believe you are correct in that he was with the right wing, and thus was not present when the decision to fire or not to fire the town was made. The railroad was of course destroyed and certain warehouses and the depot were burned, but otherwise the town was not burned. However, there was widespread looting and pillaging of stores and presumably private dwellings.
As the Federals concentrated around Atlanta preparatory to setting out for Savannah on November 15, 1864, they burned all of the towns from Roem to Atlanta -- Rome, Kingston, Cassville, Cartersville, Big Shanty, Marietta, and of course Atlanta itself. The Slocum and the corps commanders of the left wing appear to have been much more respectful of private dwellings and towns on the march itself than the troops on the right wing. Logan's command was particularly unrestrained, which clearly reflected command complicity by Logan and of course Oliver O. Howard. Hillsboro, Irwinton, and Gordon were all burned to the ground -- not only railroad and other public property, which ce3rtainly could have been considered a legitimate military target, but the private homes as well.
The ultimate responsibility of course rests on Sherman, and he was aware of the depredations of LOgan's troops, in particular. When writing to Henry Halleck shortly before his troops left on the invasion of South Carolina, he referred to the reputation of Logan's corps with regard to Halleck's desire to wreak vengeance on Charleston, mentioning that Logan would be on the right and nearest to Charleston.