Booby trapping may seem costly by itself, but it can save you several TPs in trap replacements. The best way to use it is to rig only your farthest traps—for example, the top mine in a triangle—which are the most likely to be reached by a German Pioneer.
An interesting fact to note with booby traps is that when triggered, they do not cause the explosive to go off where it is, but rather make the assault pioneer explode instead, but you still lose the Mine or TNT. Kill 25 of them with booby traps to receive the useful Interdiction Ribbon. If you try to stack multiple booby-traps on a single mine or TNT, you will see a message: "As much as we admire your aggressivity, you can´t stack boobytraps.. for safety reasons."
Tip: If you place your topmost mines no more than 100 pixels apart from each other in a horizontal line, you will effectively guarantee that no Pioneer shall ever pass them. You can accomplish this by placing only 3 mine sets and rigging the top 3 mines, as shown in the diagram below:
Formation: | | | + + + | | * * * * * * | | | Legend: +:rigged mines *:regular mines |:sides of the map