is it important to wear an undershirt under a dress shirt especially when wearing a suit?

Dress shirts feel much cooler to me when I wear an undershirt so I do it merely for comfort.

I always do, yes.   

Not unless it's cold, or you're hoping to wear the shirt again the next day.

You wear the undershirt to protect your shirt from perspiration stains. Also, if something happens and you need to take that dress shirt off, you won't be naked. 

Yes, otherwise your shirt will stink of sweat

Whatever makes you not look revealing and look decent, duh.

If you choose best answer, leave comments!

If you are concerned about your nipples showing or about perspiring, then yes;  otherwise no.