Depending on your experience, not using the clutch is much harder on the shift forks, drive shaft and rear end, than the gears. Car and truck transmissions have a neutral in between each gear. In a car the syncro either speeds up or slows down the rotating mass(including the clutch disc and countershaft) to allow the shift collar to align with the dogs on the gear. In a heavy truck, there are no syncros, thus the need for "double clutching" where during the shift, the clutch is released in the neutral position, disengaged again to place in next gear. The other choice is to "float" the shift. In that case, as you come to the shift point, throttle pressure is reduced to remove torque from shift collar, shift level is placed into neutral, rpms are allowed to drop to the proper level, and shifter is moved to next gear. Down shifting requires the throttle to be "blipped" in neutral to speed match the gear, providing you are going slow enough that the engine governor allows enough rpms for the lower gear you are looking to select.
As remarked earlier, "1-2 shift is more difficult" this is because there is a neutral there.
High performance transmissions such as Tremec trans used in Nascar and road racing, the shift collars and dogs are ground with a special lead angle to suck the collar and dogs together an lock them under power. These transmissions wear the angles down fairly quick, requiring repair after a few races.
Knowing whats going on inside, I think I will use the clutch, but that was a nice video.