I'm used to save prefs in a json file and read them from the file at the start of a script.

For you case it would be something like this:

var doc = app.activeDocument;

// try to loads saved prefs or set default prefs
var prefs = load_prefs()
|| {a: true, b:false, c:false, d:false, e:false, choice:'MN'} 

var choice = prefs.choice; // <-- get choice from prefs;
var w = new Window("dialog");
w.text = "Please Select Your Gemini Save Location Below";
var g = w.add("group");
var a = g.add("radiobutton", undefined, "MN");
var b = g.add("radiobutton", undefined, "WA");
var c = g.add("radiobutton", undefined, "CA");
var d = g.add("radiobutton", undefined, "TX");
var e = g.add("radiobutton", undefined, "Remote");
var button = w.add("button", undefined, "OK");

// set the radiobutton from the prefs
a.value = prefs.a;
b.value = prefs.b;
c.value = prefs.c;
d.value = prefs.d;
e.value = prefs.e;

var radiobuttons = [a, b, c, d, e];
for (var i = 0; i < radiobuttons.length; i++) {
    (function (i) {
        radiobuttons[i].onClick = function () {
            choice = radiobuttons[i].text;
        };
    })(i);
}

w.show();

alert(choice);

// set the prefs from radiobuttons
prefs.a = a.value;
prefs.b = b.value;
prefs.c = c.value;
prefs.d = d.value;
prefs.e = e.value;
prefs.choice = choice; // <-- save choice

save_prefs(prefs); // save the prefs


// functions to load and save prefs

function load_prefs() {
    var file = File(Folder.temp + '/prefs.json')
    return (file.exists) ? $.evalFile(file) : false;
}

function save_prefs(prefs) {
    var file = File(Folder.temp + '/prefs.json')
    file.open('w');
    file.encoding = 'UTF-8';
    file.write(prefs.toSource());
    file.close();
}

Just in case. Of course you can use a loop to set radiobuttons from prefs and vice versa if you like. Something like this:

// prefs
var prefs = {a:true, b:false, c:false, d:false, e:false};

// radiobuttons
var a = {}, b = {}, c = {}, d = {}, e = {};

// set radiobuttons from prefs
var radiobuttons = {a,b,c,d,e};
for (var btn in radiobuttons) radiobuttons[btn].value = prefs[btn];

console.log(radiobuttons)

// change values of some radiobuttons
a.value = false;
b.value = true;

// set prefs from radiobuttons
for (var pr in prefs) prefs[pr] = radiobuttons[pr].value;

console.log(prefs)