Without giving too much away, my company saves over $1 million by slowing to ICR (Intermediate Range Cruise) from MCR, so going down to LRC maybe call it another $300,000 to $500,000. We are planned by the dispatchers for all flights at MCR and then once at cruise we determine based on the schedule, anticipated arrival and approach, and other factors whether to slow to ICR (Intermediate Range Cruise) or LRC. We decide in 15-minute blocks. If we are late or up to 15 minutes early, tend to stay at MCR and accelerate to VMO-10 or whatever is shown on the flight plan for TAS at Cruise. From 15 to 20 minutes we tend to slow to ICR, and from 20 minutes on we slow to LRC. Of course, all of this depends on the sector length. We have a couple two and three-hour sectors where we might not slow down until we have a better idea of what the winds ahead of us are doing - 20 minutes early at TOC can easily become 20 minutes late if you don't think ahead.
A competing airline to mine always plans at IRC because most airports for our area of operations do not treat the Q400 like a jet, so they've found they're the ones always getting speed reductions or vectors anyways. Indeed, the "jet" performance of the Q400 is only because we can keep up below 10,000' because of the speed limit order. Above 10,000' and we're just another turboprop. I think the book says we cruise at around 0.5 Mach, but there is nothing in the flight deck as standard equipment to tell you the speed in Mach, and we're blown out of the water by most jets above 10,000'.
There are two other main ways of saving money in Q400 operations: single engine taxi and the FL270 supplement. We use single engine taxi whenever we can. Bombardier says you'll save an extra 3% in fuel by going up to FL270, but you need to install the drop-down oxygen masks for that supplement and just the maintenance costs outweigh the fuel savings for most Q4 operators.
So does it make sense to plan the way you've suggested? Sure. But you give your pilots zero wiggle room to make up time at night if you plan at LRC and only load that amount of fuel. Furthermore, night ops are where all the compounding delays from the day build up to bite you, so you don't want to be slow as you're likely trying to prevent your OTP from going out the window. Better to always plan for the higher speed, then let your pilots decide to keep going fast to maintain the schedule or to slow down to save fuel (and maintain the schedule).
Consider it the difference between theoretical operations and practical operations.