I was watching a documentary and it said that when the Wermacht shelled Stalingrad, the rubble from the buildings made it easier for the Red Army to defend. They said it slowed progress for the Germans and was one of many things that helped lead to defeat. How true is this?
From several accounts I've read about the matter it was the initial Luftwaffe bombings that mainly created the rubble in (west)Stalingrad.
Only later did the Germans start transferring heavy artillery to the city. The Soviets built up a large number of artillery batteries across the Volga and Soviet artillery on the eastern bank continued to bombard the German positions as well. The Soviet defenders using the resulting ruins as defensive positions is clearly documented, but a non-ruined city can be an equally hard nut to crack and in any case they did not have much of a choice. German tanks at some points became useless amid heaps of rubble up to twenty feet high. When they were able to move forward, they came under Soviet anti-tank fire from wrecked buildings. Soviet snipers also successfully used the ruins to inflict heavy casualties on the Germans.
Zhukov cs pioneered a technique called "hugging the enemy" at Stalingrad to negate the Germans close-air-support air-superiority there, and much of the effect of the artillery and armour-arm of the German combined arms-tactics.
But again this is a viable tactic in both ruined and non-ruined cities, roads can be blocked for vehicles either way, there is hard cover, possible elevation, there are alleys and cellars etc. From the pictures I've seen of pre-war west-Stalingrad it had very, very few buildings over two storeys high for the largest part, so elevation would not have been affected too much.
To conclude, if it made it easier for the Soviets, which seems to be the consensus on both the German and the Soviet side, it was probably only marginally so, because assaulting a fanatically defended large urban area which keeps on receiving reinforcements and supplies is very difficult, whether the city is in ruins or not.
And if it lasts long enough the ruins are inevitable.
Many sources, including Glantz, David M.; House, Jonathan (2009). "To the Gates of Stalingrad: Soviet-German Combat Operations, April-August 1942".
Plug for my favourite book about the battle: Stalingrad by Theodor Plievier (it's a novel though, not a hard history book).
More information on Urban Operations in general:
Urban Operations, FM 3-06: US Army Field Manual 3-06