I have a little MFJ QRP CW 20m rig that I plan to turn into a portable kit for SOTA peaks. I have everything figured out except for the antenna. I am wanting to make a dipole. However, I was wondering, is a Balun really necessary for this? This is all I plan to use this antenna for. (This one band, with QRP)
You don't need a balun, in that it will work and it won't hurt you. But your antenna will not work as well as it ought to.
When you are transmitting, the effect of a balun used to connect a coax (unbalanced) feed line to a dipole (balanced) antenna is to prevent the RF from the transmitter from returning down the outside of the feed line and go entirely into the antenna instead.
If you don't do this, then your feed line is effectively part of your antenna. This changes the pattern and means it has a part close to ground — so you've put up an antenna that is not the shape you intended, isn't resonant where you intended, is more lossy, and is partly made up of the transceiver, your body, and any other connected equipment.
There are two things you don't need to worry about as much because you're QRP portable:
You are not transmitting enough power to be dangerous. If you were, then the issue is that the RF returning down the feed line would make the case of your transceiver (and anything else sharing the same ground) “hot”, possibly causing RF burns when you touch it.
On the other hand, you're not transmitting very much power, so you probably want to make sure every watt is getting out as well as it ought — which means you don't want it going places that aren't efficient antennas, which means using a balun.
If you were not on battery power, then noise coming from the power line could come in on the ground, up the outside of the coax, down the inside, and into your receiver. Even so, any noise sources among your equipment will interfere more than they would if you had a balun, but I'd guess they're unlikely to be strong enough to make things worse.
It is also common to include impedance matching in the construction of a balun. This conceptually a completely separate function, but if you're building a balun (other than the simple choke type) you might as well include it; typical amateur equipment is designed for a 50 Ω impedance whereas resonant dipoles have impedance around 70-73 Ω depending on surroundings.
Impedance matching prevents the transmitter from being exposed to high SWR, reduces losses in transmitting (assuming that the balun is located at the antenna end of your feed line, not the transceiver end), and acts to some degree as a band-pass filter for received signals. Again, none of this is necessary, but it all helps.