I'd say put them on the ends of the big X in the middle, provided that the length across the X is enough to hold a whole train.
DCC or DC, a reverse loop will always create a short circuit. Think of it this way.
Each rail has a different polarity. If you loop a length of track back into itself (so that a train travels down the track one way, turns around, and comes back on the same track in a sort of P shape), the two rails connect to each other and you have a short circuit. Looking at what I've described, you'd have one solid rail (not counting connectors) going all the way around, from connection to connection.
To keep this from happening, you need to electrically isolate the loop part of the P. To make this simple, say you're using DC. You've got a switch that changes the direction of the current in the top part of the P, and a switch that changes it for the straight part. You have gaps on both ends of the loop, where it meets the switch. (Think of it as a Y with the top two prongs looping back to each other; the switch is at the crotch of it.)
Whatever way you set the switches, you can only make the current match at one set of gaps. So you bring a train in from the straight bit, and change the current in the loop (if needed) to match up the gaps that the train will hit first. It crosses the gaps and enters the loop. Before it can cross the other set of gaps (which aren't matched), you reverse the current on the straight bit. Then the gaps are matched, the train crosses them, and it leaves going the other way.
DCC is easier. You just hook the loop part to a reverser unit. The engine enters (again, imagine it's the P shaped setup), and the points aren't aligned right. So there's a short. Before that short can become a problem (or really be noticed at all), the reverser reverses the current to the loop, matching the gaps, and the train rolls through. When it gets to the other set of gaps, the same thing happens. Since DCC trains don't care which rail is which, the train continues forward, and leaves the loop onto the straight bit, again, without any indication.
You just have to make sure the whole train can fit into the section with the reverser on it. Otherwise, the train will hit both sets of gaps at the same time, and again, you have a short. Of course, on the P layout I'm using for this example, this scenario wouldn't be possible (the train would hit itself on the switch).
I mention this because if you isolate just the X, and try to run a train longer than the track across the X, the reverser will get confused. If the train's short enough to not hit both ends at once, you're good to go.
Unless you plan to run two trains across the X at once (daredevil style), you can probably get away with wiring both legs of the X to one reverser. Otherwise, hook both to its own reverser. Wire the rest normally.
(You could wire the X straight to the cab unit, and wire the outside loops to a reverser. It would work just as well, it's just not quite as logical.)
Or, you could avoid the short entirely, if you move the crossovers from the bottom to the left side, and eliminate the straight track directly above and below the X. This would however take away your ability to turn a train around.
If you read all that, you deserve a medal (or a cookie anyway). Hope this helps somewhat.
