Yes it is and important that you do it! Read the 45-70 200 yard post next to this one and you'll see that filling the chamber is the most important factor in fitting bullets to 45-70 rifles. I'm talking lead of coarse, but it also applies to jacketed! The constrictions will lap out easily with the LBT lap kit, and shoot like a match grade barrel. Throating of your Ruger is standard with all rifle makers, so far as rifling starting just off the case mouth, and 459 diameters are not uncommon except in the premium match rifles like Sharps.
Any machine work done on a barrel after it is rifled changes the internal dimensions. Be that contouring, drilling, tapping, barrel bands, fluting, or making them octogon. Internal dimensions are changed and target results are negative. Lapping cures the problems. I have found that it is pointless to try to shoot cast without lapping, even old well worn barrels, but especially new, and I've never heard of a match grade barrel that wasn't improved by it.
