Actually i dont know how you would get to calm tranquil and pleasant (see the anapanasati sutta quote above) except thru some degree of relaxation. As Kim pointed out, the usual meditation postures with your spine unsupported, wont allow you to actually sleep (long or deeply anyway) and if you are living in 21st century western culture on planet earth, you are probably experiencing stress levels previously unknown to the species. So yeah, go ahead and relax
EDIT: I had to come back to this, because the single biggest hurdle in my meditation practice when i started was that i didnt relax. So i wasted alot of time with typical goal oriented western behaviour. If i could go back in time 15+ years, the biggest thing i would change about my practice is that i would relax more. Im sure someone at some point told me to relax and i didnt listen and the result was alot of wasted time and effort.
If you look at the basic instructions above, a calm bodily and mental fabrication is stressed and pleasure is also stressed. The only way you get to calm and pleasant is to relax afaik.
Also think about the people the buddha was giving these instructions to, these werent stressed out westerners with a million things clamoring for their attention, these were guys who basically had only one task a day, walk to town for lunch. Imo if the buddha were preaching today, to westerners, he would postively underline relaxation.
“The truth knocks on the door and you say, "Go away, I'm looking for the truth," and so it goes away. Puzzling.” ― Robert M. Pirsig