Better to use FIXED BUT MX at one end unless you are expecting some torsion in the member. Releasing MY would mean release of minor axis moment. In your case, since you do not have loads along the minor axis, it may be ok either way. But here is what I would generally use for cases like these
SUPPORTS
1 FIXED
2 FIXED BUT MX
SUPPORTS
1 FIXED
2 FIXED BUT MX