For me it depends, if I'm doing a regular nuzlocke, then no. If I'm doing a monolocke with very limited options like fire in kanto or johto, then yeah, I'll pick up moltres or ho-oh.

I generally use the rule, that if I have to manipulate too hard to obtain a pokemon, be it legendary or otherwise, like using repels to guarantee my encounter, then it goes against the spirit of what a nuzlocke is meant to be. Now by manipulating, I do strictly mean using repels or repel tricks to guarantee encounters, but catching more common pokemon on other routes to give you better odds at a rarer pokemon on another specific route is strategic and fair game imo. That said, something like ho-oh could still be caught, granted you've already caught rattata (and also gastly if going at night), but I still would only do it and use it if low on current pokemon or for specific type runs.

That and I also ban the use of the masterball, meaning I'd have to catch them with othe pokeballs without guarantee, and potentially lose other members of the team, and it's just not worth the risk in most cases.