Ok... this fashion faux pas REALLY irks me. Ladies repeat after me, "I will NEVER wear a shell under a strapless tube dress to make my outfit tznius or under a spaghetti strap dress." If I see you in the streets like this - RUN. Cap sleeves, sleeveless and V-necks work great with simples tshirts underneath. I'm not quite sure about the off to one shoulder look. For now I say, as long as the shirt under matches the dress exactly - it's ok. Below are some great options:
This Tadashi 'Windowpane' Organza Dress for US$388 will look great with a black 3/4 length Tee and you can style it with a wide black patent belt.
This Adrianna Papell Rosette Shirtdress for US$182 can be worn with a white, pale pink, or light grey shell for a great summer look. Avoid black with it and have a light pair of shoes to accessorize.