I'd imagine that most of us play Scheme pretty similarly - use it to topdeck good actions when possible, putting Scheme itself on top only if you can't topdeck anything else worthwhile. This can actually be less effective than topdecking a Scheme instead of your good action - namely, this occurs when you are close to the reshuffle.
Suppose a case where I have 16 cards in my deck - all treasure, plus a Scheme and a Militia. It's the end of a turn where I just played both the Scheme and the Militia, and I'm at the very bottom of the shuffle, with no cards left in my draw pile. I imagine most people here would just automatically topdeck the Militia. You'll actually play it more if you topdeck the Scheme. Putting the Militia on top, you have a 4/15 chance of drawing your Scheme with it, and getting to play the Militia twice during this shuffle. If you miss, you only play Militia once this time around. Putting the Scheme on top, you have a 5/15 chance of drawing the Militia with it and getting to play it twice. Even if you don't, you can just topdeck the Scheme again and have a 5/10 chance of getting the Militia and still playing it twice this shuffle. Worst case, the Militia is in the last 5 cards and you only play it once this shuffle. Overall, you have a 66% chance of playing your attack twice during this shuffle if you topdeck the Scheme, compared to 27% by topdecking the Militia.
This happens because topdecking at the bottom of a shuffle might not have any effect - you might have gotten lucky and drawn that card anyway. You're better off topdecking your topdecker, so that you can play it early in the shuffle when it will certainly have an effect. This seems counterintuitive; I usually play my Schemes on autopilot, only Scheming Schemes as a last resort (or to enable Conspirator or KC shenanigans). This gets a little more complicated in reality because by not topdecking your attack, it might miss the reshuffle (playing it 0 times this shuffle), but this gets less likely as your deck gets bigger. Plus, timing of attacks can be important - sometimes it may be better to play an attack earlier rather than an extra time. Overall, though, I think the right play might be to topdeck the Scheme if you're about to shuffle, and hope your attacks get drawn at the top of the deck.