Okay, so this answer assumes that you've read the other FAQ called "Do I Need a Preamp?"
-- if you haven't read that one, please do.
Now, the straight answer is that it may
help, but it's really hard to say for sure.
It can depend on your pickup; a pickup mounted/clamped/glued to the surface of the bridge (like a Fishman BP100, K&K Double Big Twin, etc.) puts out less sound than one that is under pressure; this means you need to have more "makeup" gain to bring it up to performance levels. This added gain can increase the likelihood of feedback, since raising the incoming signal also increases noise and makes the pickup more "sensitive" - and thus, more likely to feed back, since that increased sensitivity is more likely to pick up the output from speakers and amps onstage.
Also, like we mention in that other FAQ (you DID read it, didn't you?) piezo-electric pickups are commonly known for having a "quacky" response when a buffering preamp isn't used - most players prefer at least 1 megohm (5-10 can be better, depending on the pickup used) to make such a pickup sound good. Not having that impedance buffer can create some weird peaks and valleys in the tonal response of the pickup, which you end up trying to fix with all sorts of crazy EQ correction
"Correction" is the key word, here - it signifies the difference between radically altering the EQ controls to just make the pickup sound "decent," rather than "enhancement," which is just making subtle tweaks to improve an already reasonably good sound. Doing this sort of extensive correction can also open up avenues for feedback; by over-compensating with EQ at certain frequencies to get a decent tone, you may also be unintentionally boosting particularly "touchy" or "hot" frequencies for feedback as well.
So a preamp that increases gain and properly buffers the signal may
allow you to run a lower input gain, and flatten the EQs a bit, both of which may
help reduce feedback.
But there's not really a guarantee, since there are so many other variables and causes for feedback.
If you've bought a pickup or preamp from us, we included a bright pink sheet of tips, which has about a page's worth of tips for reducing feedback and getting better sound. Don't have that? Email me
and I'll send you some tips!