2013 LaCrosse Throttle Body cleaning

I noticed that my 2013 LaCrosse 3.6 L is not getting the mileage it was when I first bought it.
The FSM provides instructions on cleaning the throttle body valve plate for deposits.
My car has about 27500 miles. The FSM says this cleaning procedure should not be performed on vehicles with mileage under 50,000.
It also says that following the procedure you should perform the Throttle Body Learn procedure using a scan tool to reset all learned values back to zero.
I don't have a scan tool. Can I not just take a few drive cycles to do this?
Any tips on cleaning the throttle body?

I doubt that throttle body cleaning will help your gas mileage...

If you want to do it, with a rag soaked in carb cleaner, open the throttle plate, clean the edges of plate, and the mating sides of the bore.......you do this with the key out of the ignition...

Now remove the negative bat cable....then the pos cable....now hold the two cables together, for at least a few minutes(I use a jumper cable and attach it to both ends of the cables....this resets every module in the vehicle)....

Then, reattach the positive cable first, and then the negative cable.....a couple drive cycles and you are good to go.....

When you say gas mileage has declined, is this based on readings from the DIC? What are the differences in the readings?

Have your habits changed? By that, I mean, do you do more local driving now than highway?

The mileage readings are based on the DIC. When I bought the car a year ago I could get 32 mpg while on the highway by setting the mpg average to zero at the beginning of the trip.
Last weekend, I could only get 29 mpg doing the same procedure.
Perhaps I am nitpicking and perhaps the outside temp could cause the change. It was in the 30s last weekend. It may have been warmer when I got the better mileage.
If the throttle body cleaning is not going to help, could the injectors be dirty?