Some heads have +-90° tilt range, e.g., Ronford Baker, Oconnor, some Cartoni and Vinten, while some have +90°/-75°, e.g., Sachtler. In binoscope secarios, my advice is always looking for the +-90° models.
There are many fluid heads offering full 90° up, but "only" 70° or 80° down. Personally, I don't have experience with fluid heads, so I would be interested to know more on your +/-90° model advice. As long as 90° up is given, what, with astro use in mind, would be the disadvantage of a reduced down range?