Based on mike's critique here, I did reconsider the +1 to -1 scale for bonuses and penalties. What I have decided is that the bonuses are right for now.
Bonuses and penalties to rolls of +1 to -1 works for Mythical Journeys is because a lot of checks are on 1d6. A +1 on 1d6 is equivalent to 16.67%, a little more than up to +3 on 1d20, which is up to a 15% bonus.
The main checks that are done on 1d20 are attacks, saving throws, and ability checks. Ability checks are rare because a lot of checks are subsumed by specific 1d6 checks. Only dexterity affects attack rolls, and only for missile attacks. Dexterity also affects armor class. Strength does not affect attack rolls, only damage rolls. Wisdom affects magic saving throws. Ability checks look at the whole ability score, not the bonus or penalty.
The main checks on 1d6 are surprise, sneaking, searching, and feats of strength such as bend bars/lift gates. These are checks largely important within the dungeon.
The checks on 2d6 are reaction checks. Charisma affects reaction checks. It should not overwhelm the dice though.
In my opinion, stats should matter only a little. It is just as important that they be aids to the imagination as they are game mechanics.