I'm considering using an Arduino Uno board with a rotary encoder to use for MAME arcade games. Is it necessary to program these boards prior to being used for that purpose? Thanks in advance for any help. I'm not a programmer and just want to know what to expect if I get one of these boards.
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The simple answer is yes, the board needs to be programmed for whatever task you want it to perform.
There are varying degrees of ‘more complex’, but a starting point you need to decide is whether 200 hours of your single-minded dedicated time is worth more or less than buying a completed gadget.
USMC0352:
I'm considering using an Arduino Uno board with a rotary encoder to use for MAME arcade games.
I had to look up 'MAME arcade game' as I had no clue what this meant. My impression, from a quick look at Google results, is that programming an Arduino, or indeed any kind of micro-controller, for this purpose would very much be an advanced project. Consider how much programming knowledge you already have and how much time you want to spend leaning.
PerryBebbington:
Consider how much programming knowledge you already have and how much time you want to spend leaning.
I think that was detailed in the OP, and conservatively elucidated by lastchancename. 
My impression was that he was planning to run MAME itself on a proper computer (which is what he should be doing) - but wants to make a custom controller with a rotary encoder. And this would connect to the computer running MAME through USB.
You don't want to use an Uno for this - you want something that has native USB and can be programmed to act like a keyboard/mouse/joystick. A leonardo or - probably better due to size - a micro/pro micro is a great choice for this.
You will need to write the code that reads the rotary encoder and uses the Keyboard or Joystick libraries to act like a USB keyboard or joystick (as your needs dictate). I have actually considered doing something like this myself - not with MAME specifically, but for playing Tempest (both the windows arcade version, and the new one) with that wheel. I think the software part is easy, but making a satisfying wheel is non-trivial from a fabrication perspective, since it needs to have some weight to it while spinning freely, in order to feel right. And in any case I put a bunch of equipment in front of my big monitor, so I couldn't use this if I made it anyway.