The thought that circulates in the antique truck world is that you use the same gear going down a hill as you used to go up it.
For old trucks like mine, I sometimes have to get into first gear for some hills so it's a slow 10mph grind. I'll go a little faster downhill, but I need to keep it slow to preserve the brakes and keep them working.
For parades, the granny first gear in Lurch's transmission is perfect. I let him idle in first gear and we're keeping pace with the folks that are walking in the parade. ;-)
Dean