Wondering if there's any benefit/speed difference by plugging an SSD into the cable where the optical drive was or if one of 4 hard drive bays connection is faster on a Mac Pro 2012 (Westmere)?
Where you physically store your SSD should not matter as long as it is secured properly (unless you plan to just leave it hanging on the cable.... which I've done before and no noticable issues... except when the PC is moved and the SSD slides here and there inside the PC).
Caveat: The above is fine because it is an SSD.
If it is a spinning disk (HDD) - then securing it properly matters as HDD spins the disc and cause vibration. HDD not secured properly will vibrate (at whatever rate) and could slide off. Or in worse case, cause the case to vibrate and/or reflect the vibration to the HDD - in turn could cause problem with your HDD.
Reference for vibration/sound is bad for HDD is on this link https://www.youtube.com/watch?v=tDacjrSCeq4&feature=youtu.be