It probably also depends on what type spindle and crank you have. Square taper cup and cone you have to pay lots of attention and there are offsets thrown in too sometimes. Cartridge BB's with square taper you have to pay some attention more closely than ISIS, Octalink or other. With a two piece crank, more common on road bikes, there is even less need to worry as long as you get the compatible BB for both your crankset and BB shell.
With two piece cranks, as long as you keep everything within the proper group and you aren't building a franken bike or such then spindle length and chain line is something the entire group of components take into account by design and aren't something that needs to be in the forefront of the purchasers mind. If you get mixing new and old components it might be something to be aware of. But in my conversions of square taper cup and cone to modern 2 piece road cranks, the only need was to know the BB shell width, diameter and threading. In all cases these were BSA threaded shells on the bike's that I converted.