It varies, sometimes the same artist draws the same outfit both ways.
Perez (and others) usually had Dick wear a short sleeved green leotard under the red tunic, but did draw his outfit as separate T-Shirt and briefs... But only if the plot required it of course
Also, his first Nightwing outfit was FAIK a one piece, then he draws this;
I think the rule is "Draw whichever makes him look sexier"