How Optional are GRE Subject tests?

[WCL]

So, like the title says, how optional are the GRE Subject tests?
Many engineering and interdisciplinary fields such as materials science state "optional" GRE subject tests, but I wonder how optional they actually are. What weight is usually put onto tests?
What if the scores are mediocre, about a 50 percentile? Will that hurt the the chances of admission if the other credentials are superior? Should mediocre scores only be reported if the other areas are lacking? Will it raise many questions since that's not the intended field? Obviously answers are quite subjective, but I lack any experience in this, and anyone who has gone through this or has been on an admissions committee would be helpful.
Thanks.

PS: Yes, I've gone through the older threads, some are close, but many don't apply to subjects that don't specifically have a subject test. I've especially gone through Zz's thread where he says that it's often many schools' only comparison, but that was for a specific subject tested by the GRE.

 

[member 428835]

sorry if this answer is too bland, but to know for sure how optional the gre is, email the faculty of the school(s) you plan to attend. everyone here on pf can tell you their own experience and preference, but when push comes to shove it will be the admittance committee reviewing you app. thus, email them directly.

hope this helps