As it is to be worn underneath your top or dress, you needn't worry about armbands, neckbands, or even hemming. Knit fabrics do not fray, so leaving your edges raw will only make for a visually professional finish. Merrick does provide optional instructions on hemming edges if you choose to hem, but it is entirely up to you.
Some tops and dresses will look better when the under tee is a complimentary color (for example, if your dress is navy you might want to consider making one in navy). A few of my picks from the store:
~ Heather & The Girl Charlee Team