In the case the a CommonJS module only need to return one function, is it good practice to assign it directly to the Exports object? (as opposed to assigning it as the only attribute of the exports object)

For example (imaginary log.js module):

module.exports = function(text){console.log(text);}

usage:

var log = require('./log');
log('something');

Yes. I often use this practice myself. It's also a good fit with the revealing module pattern, e.g.:

module.exports = function(db) {
    return {
        get: function(id, callback) {
            // db.get(...)
        },
        save: function(obj, callback) {
            // db.save(...)
        }
    }
};

It's also widely used in connect (e.g. csrf module) and express (e.g. route).