It is true, however, without undershirt or bras on, I can apply smooth, continuous gliding strokes to the body, which would feel more relaxing, if you are comfortable. I get permission from clients to unhook the bras only when I work on the back, and put them back. However, if you are not comfortable with this request, you should say no.
Regarding underpants, I don’t mind if they are on or not. Male clients tend to keep them on maybe because they don't want to embarrass me, but I am professional, so I won't be uncomfortable. If clients wear boxers or briefs, I ask them to pull them up toward the hip bone when I work on their upper legs. Without underpants on, I can apply continuous strokes up to the top of the thighbone and the buttocks to be able to address low back pain, sciatica, knee pain, and tight quadriceps, hamstrings, and IT bands (a thick band on the side of your thighs).
When clients, whom I have seen some time, have sciatica or low back pain, and I think it will be more beneficial for them to receive work on their buttocks without underwear, I explain it to them and make sure if it will be totally comfortable for them. I recommend you try without underpants on next time if you are completely comfortable with the idea.
You might also want to read the related post "FAQ: Does a massage therapist work on my buttocks?" and watch the video.
FYI - Draping is required in the state of Florida, so even if you are without underwear, the gluteal (buttock) cleft must be covered all the time, and only one side of the buttocks is undraped at a time.