I am somewhat new to lutherie and am making a guitar right now. I am making the neck out of mahogany, which is a pretty hard wood. My question is, should I put in a truss rod in the neck, or will it be alright without one?

You have not said what kind of guitar you are making, is it a 6 or 12 string and is it nylon or steel string, and how long to you want the guitar to last??? If you are making a nylon string guitar, then a truss rod is probably not required, if you are making a steel string guitar then a truss rod is recommended to help keep the neck supported and to be able to adjust the neck should it ever need it. If all else fail, error on the side of safety and install a truss rod.

I am making a steel string acustic guitar. What do you think about an adjustable truss rod versus and non-adjustable one.

If you are making steel strung you MUST put in a truss rod, otherwise you will pull the belly off! The tension of steel strings is amazingly high.