Kyle,
I'm a little hesitant to give you my "story" on this because I had tonsil cancer, not tongue cancer, and my experience appears to be very, very different from others. But you asked, so here it is.
The nurse recommended I get a PEG prior to radiation, but my RO said I should try without it and get it later if needed.
I opted to wait. I went through a big problem with being able to eat fairly soon after starting treatment, then got on a diet I could tolerate for 3 meals per day (Boost Plus and egg-beaters) and which gave me about 2700 calories per day.
I ended up taking steroids (unrelated to eating issues), used the magic mouthwash at meals, and took a codeine-tylonol syrup, all of which I believe helped my abilty to eat. I also drank a lot of water and some tea.
I lost very little weight.
You can see the other very bad experiences above. In light of all the bad experiences, it is worth another discussion with your doctor prior to making a final decision.
Best wishes,
Chris