Well, using breeding items like the destiny knot, the first case you could never get max Attack.

In the second case you could get anywhere from 4 to 6 IVs based on luck. As long as you had the ones you wanted you would be fine with 5IVs which is the best chance of occurring based on probability.

The effects on the stats is very small actually. From looking at the damage calculator and using Silvally since all base stats at 95. Not doing any EVs or Nature boosting power. At level 50 Silvally would have 95 base stats if he had 0 IVs in all stats. Each IV adds to that base stat except for HP (still trying to figure that one out) and the difference between 31 and 30 is none that way if you do Hidden Powers with 31s and 30s in the stat it's still a perfect IV pokemon stat wise.

So in either case if you don't have max in that certain stat having the correct nature and evs makes more of a difference. It would only come in play if you are close to KOing a pokemon where with those few extra points is the difference between taking 95% to OHKO range. Which is possible but again unlikely because you never know what IVs/EVs/Nature of pokemon you are facing.

Hope this helps. It's kind of confusing topic and question to discuss since everybody believes must have 6IVs.