Aside: the slacking the strings thing, never got it. "They" say you are supposed to do that. Who are "they" (and Dan Erlewine doing his imitation of Dan Ackroyd imitating Julia Child is not a good enough "they"

)? Here's a hint - I worked for 5 years at Bass Central, and in that time I received (unpacked & QC'd) literally thousands of brand-new instruments from dozens of custom high-end boutique bass builders. EVERY. SINGLE. ONE. WAS. TUNED. TO. STANDARD. PITCH. when it arrived. Every one. If it's good enough for the guys that build 'em, let's go with that version of "they", hmmkay?

There is a reason why, for the record, and the tl/dr version is this - a proper neck setup is a balance between the string tension and the trussrod tension, and slacking the strings, without taking all of the tension off the trussrod will potentially force a back bow into your neck, not to mention completely foul up the nice setup on the bass that the build did just before shipping the bass - whether you loosen the trussrod or not.