I am a big proponent of the all-in overbet with the nuts on the river at the micros. The reason being, you're not getting enough extra folds to make it not worth it.
Say we have a pot of 40xbb on the river. I can bet 15xbb and get called 80% of the time, 25xbb and get called 60% of the time, or 60xbb and get called 30% of the time.
80% of 15xbb = 12xbb
60% of 25xbb = 15xbb
30% of 60xbb = 18xbb
And, those numbers are pretty conservative. Say someone calls a 3/4 pot bet 50% of the time (0.375 pot return). Are they really going to fold to a 5/4 pot bet more than 70% of the time (which is our break even point). You think they're folding that much more often? Usually not. This is especially true if the 5/4 bet is an all-in and the 3/4 bet is not. The all-in bets look "fishier" than smaller bets.
Basically, if you double your bet size (say from half pot to full pot), they need to call less than half as often as they were for it to be a mistake. If they call 60% of the time for a half pot, they need to call less than 30% of the time for a full pot to make the half-pot better. Generally, ranges don't fall off that quickly.
In hold'em, I err on the side of value more often than not. The only time I go small is when I think they are very unlikely to call anything higher, and then I still aim for the highest amount that I think can get a call, even if it is 1/8 pot. My bluffs are also based on what size bets I think they call and how often. Except there I tend towards the lower end, because the extra amount bet isn't buying me enough extra folds.
This exploitable play works well at the micros, because most people aren't paying attention or asking questions about your river bet sizing. They see that you've bet a variety of sizes with a variety of hand strengths (sometimes large with the nuts, sometimes small... sometimes large with a bluff, sometimes small), and don't connect it with the strength of the opponents range, not your own hand.
Edit: And, by all-in overbet, I meant OVERbet. I have shoved 5 times pot on the river when I expected the person to have a very good second best hand. And, I have gotten calls way more often than I should have.