The preamp can do a lot for your sound, even when the pickups are not amazing, as it can compensate for a perceived lack of low end or midrange or whatever...
At the same time, like you say, if you start with a good sounding pickup... everything downstram is easier.
But there is no fast rule.
I had a cheap Jazz with cheap but ok pickups that sounded amazing once I installed a J-Retro preamp.
However I doubt you'd get much out of a preamp if the pickups are totally uninspiring.
In some cases, like my OLPs, a pickup change improved it so much that I didn't need a preamp, and when I did use a preamp it just made it more versatile and adjustable (which is the main purpose), but it did not sound "better".
TC RH450 #8, MM Stingray #153, EBMM SUB #15, Warwick #325, OLP #13, G&L #411