That is the rule to live by. Violate it and you may die by it. Never have two different ammos at the bench at the same time. Never.
There are other issues to deal with as well. For instance, it might not be a good idea to own a 7mm-08 AND a .260 Remington. The 7mm-08 probably will not chamber in the .260, but the reverse can happen. Probably not life threatening, but it would definitely be a hunt killer if you picked up the wrong ammo. I used to pick up .308 and .30-06 cases to use with smaller bore cartridges based on these rounds. I don't have either a .308 or a .30-06, but I can make now .270 or .25-06 cases from .30-06 and .243 or 260 Rem from .308 W. This is now an issue because I do not need two different calibers in the house with the same headstamp. I'll be throwing out the cases with .308 and .30-06 headstamps; only brass with the correct headstamp going forward.
EJ