I would recommend using 1 36lp boiler over 36 1lp boilers. One, you would still need to make a firebox for each and every 1LP boiler, two, the number of 1LP boilers you would need in the long run, wouldn't scale well. You would have 36 separate multi-blocks computing steam generation and fuel values instead of 1 doing the same thing.
There was a thread a long time ago about the most efficient boiler setup for a given amount of fuel. The LP boiler (vs HP boiler) won that test. Now, a lot has changed in how the boilers work compared to before when that test was run, however, I think it is kinda missing the point a bit. If you are using something non-renwable (like buildcraft fuel), you do want to be as efficient as possible. Once you switch to a different renewable fuel source like charcoal, efficiency really ceases to be an issue. Instead, you need to look to the amount of space it takes up and weather or not your tree farm can keep up with the amount of fuel you will need. Eeking out a bit more usefulness out of something fully automated really doesn't matter.
