Mind you, I only talk about 1E and OSRIC on this blog and to be perfectly honest, I consider UnEarthed Arcana to be 1.5 which means it is not considered as 1E in my POV.
Ok, now I can go on. Most DM's are aware that in a dungeon setting, when you are rolling to determine if a Random Encounter (Wandering Monster in a dungeon) occurs that 1d6 is rolled and the RE takes place on a roll of 1.
However in an outdoor setting, and I practically live for outdoor adventures myself, the chance of a Random Encounter is different, depending on the type of environment. In the case of being outdoors, you can use 1d20, 1d12 or 1d10 to determine if a Random Encounter occurs.
It the party is traveling a well populated area, 1d20 is used. If in a sparsely populated but reasonably scouted area, 1d12 is rolled. If the party is in a wilderness or unpopulated area, then 1d10 is rolled to see if a Random Encounter occurs.
But let's not forget that when determining Random Encounters, not only does the population level matter, but the type of day and landscape as well. Determining a Random Encounter in the forest in the noon time is not the same as checking on a Plain at noon and checking a Plain in the morning as opposed to a Plain at noon.
I don't worry too much about being chained to the book as a "do or die" thing. It's my game so make my own calls as I see the situation calls for.