Big truck transmissions are not synchronized, so the preferred method is double clutching, but, mostly everyone just straight shifts. It's buttery smooth once you get the hang of it. You can even downshift without the clutch.
It's not good to do on cars though. Straight shifting will put unnecessary wear on the dog teeth of the synchronizers. This is because you are basically pushing the shift lever against the next gear waiting for the synchros to mesh at the correct RPM. It's not good to do and will definitely cause pre-mature wear.
If you lose your clutch though, you can straight shift to get where you need to go so you don't have to call the for the hook.
Also, if you do ever have to drive a vehicle with a worn synchro, you can double clutch to avoid the grinding.