COMBAT ENGINEERS --- NEVER LEAVE HOME WITHOUT THEM
I prefer to use the term Combat Engineers (CE) rather than the cumbersome "pre-charged engineers" (Sappers or Seabees would do equally well). I have found them absolutely essential for major offensives or Blitzkrieg, both for their offensive and defensive capabilities. They let me "stretch" the available engineers by enabling a single CE to do the work of two or more regular engineering units and to do it at once.
Offensively, CE units make it possible to immediately build a road on river terrain or change a road to a RR on any kind of terrain. For example, if there already is a road through forest, hills, river valley or any other difficult terrain, it takes at least two turns to build a RR no matter how many regular engineer units you stack. With CE units you can immediately build the RR needed to get your attack units to the objective without loss of movement points.
Once you have finished the attack phase, you will almost certainly have stacks of immobile, damaged attack units outside cities. If you use CE's to build fortresses for these stacks, put one or two units with high defensive values in them and park a bomber on top, then your vulnerable damaged units can only be attacked by enemy fighters and cannot all be wiped out by a single, successful enemy attack. In situations where a strong enemy reaction is likely, I have used CE's to transform the terrain to forest in order to give the stacked units an additional 50% defensive bonus.
A few general suggestions about CE. Sort out the ins and outs of stacking engineer units for "charging" to get one CE unit with 2 charges and one engineer unit with no charges in a single turn. During charge accumulation, park the units on a railroad to avoid loss of movement allowance when they are moved to where they are needed. Also, order them to Transform Terrain rather Mine or Irrigate to minimize chances that they can complete the task before you want to use them. Before saving the game or ending a turn, center the map screen well away from the area where the CE are in preparation by clicking on another of your units or cities. When a saved game is re-loaded or the next turn begins, the first units the computer will "process" tend to be those on the map screen. You don't want any "processing" before you get a chance to activate any CE units you want to use.