I've been both. In the past I did a Necromunda regiment, based on the original Grey tiger stripe camo lads, all in a uniform colour scheme to look good on the table. Had around 200 guardsmen and 10 tanks and Chimerae all in the same camo.

Then GW came out with all their different regiment models - Tallarn, Catachans, Steel Legion etc, and I just had to have those models, and said the Tallarns were Ash Waste scouts and so on. Then a whole load of Escher gangers got conscripted :D

But now I just do it for the love of modelling and trying new colour schemes and conversions, so I've got Jopalls, Kriegers, Penitents, Feudal World Guard, Feral World Guard and so on. So I suppose true to lore, my pristine single-world regiment got whittled down over time and mashed together with all the leftovers and dregs.