No machine would run constantly (unless it actually had something to do), because they'd only be supplied power when the Has Work condition is true. Not Power Requested.
Now, if you had a machine constantly doing something (dunno..a pulverizer with a quarry input, or making sand), then you would be better off with it on a distinct power line (the new plugs are good for that). That's a design consideration that has to be weighed, obviously, but the system I described should not allow machines like a rolling machine or thermionic fabricator to request power if they didn't have work to do. There is still waste, but it is mitigated, and only present when work is being done, or has recently been done.
My tree farm, squeezers/fermenters (haven't figured out a good number of them to keep up with the tree farm output) run constantly. They are always producing more saplings/apples/wood from the tree farm. I also have 8 Igneous Extruders feeding 8 Pulverizers feeding sand into 8 Powered Furnace to make glass to make buildcraft tanks to hold all of the apple juice and biomass produced by the above.
If I do happen to go into genetic bee breeding I will also have a constantly running Genepool producing BNA. Which I haven't decided if I actually want to do that.
On top of those constantly running machines I will be supplying power to an AE network to store all of the Saplings/Apples/Wood, at least until such point as I figure out how many Squeezers/Fermenters I need to process everything that comes out of the farm as it comes out.
Eventually I will also have a Centrifuge running constantly processing all of the output from bees. I will also probably have a squeezer running constantly off of that producing honey.
I may or may not also have a Magma Crucible running constantly at some point producing lava from netherrack.
It's hard telling what all else I will eventually be doing that will have things running constantly. All of this will be ran from one central power station. Every thing else will be attached to the main power line running off of the engines.
I personally do not wish to have engines placed all over the place with liquiducts running all over to supply fuel. I would rather keep just one area devoted to power generation.