I think you don't need a 5x31 pokemon or a 6x31. But the closer to 31iv the better. So if you can't breed your mons 5x31 you can make some more cheapier and use it normaly. And in the case of 6x31, i don't recomend it when the pokemon its not a mixed attacker, like salamence, metagross, lucario.
Take a look in a example for Ivs mattering or not:
252+ Atk Swampert Rock Slide vs. 252 HP / 252+ Def Volcarona: 156-188 (81.2 - 97.9%) -- guaranteed 2HKO (31 IV Def and HP Volca)
252+ Atk Swampert Rock Slide vs. 252 HP / 252+ Def Volcarona: 172-204 (89.5 - 106.2%) -- 37.5% chance to OHKO (15 Def IV Volca)
In one case as you see, volca can take a rock slide from a swampert( its not a stab move ik, but is only for a example)
In the other case it have a 37.5% chance to get knocked out by a rock slide due to not have 31 iv on DEF.
A pokemon that can be used and its not even 3x31:
Try always put the ivs the closest to 31 if you want a fast breed or a cheapier breed. And put the 31 in the main ivs.
AND, as i said, when you have the chance, make your pokemons 5x31 if you can. They'll do the best results.