An interesting thing happened the other day, was putting a set of Labella flats on the testbed bass and when pre-stretching the strings (by no means super hard, I’m careful) the G string ball end broke off.
I emailed La Bella and they kindly sent me a free one but told me that I should never pre-stretch any of their strings.
Found that a bit odd.
I’ve never pre-stretched my strings. Didn’t even know that was a thing. I put them on and do some normal guitar bends and such and then re-tune. Then just re-tune daily until they settle.
Tank is a professional guitar tech who travels with bands as well as a bass player. Check out how aggressive he is in stretching.
I never ever pre stretch strings, regardless of the brand or type
I’m glad LaBella took care of it.
Tank is why I started pre-stretching!
Bad Tank! Bad!
John Carruthers at Elixir strings also stretches his a fair bit.
That seems like an odd caution.
I always pre-stretch my strings and have never had one break.
It’s a pretty normal practice I would have thought.
(Disclaimer, I’m a Bass newcomer so my experience is primarily on 6-strings)
Then again, I’ve never bought La Bellas.
And I must admit, their caution to you makes me much less inclined to do so.
FWIW, I’m a newbie as well, but I have changed strings twice, once on each of two basses, and I stretched both (one set of flats, one set of rounds).
I always stretch guitar strings and they stretch quite a bit, but never found it necessary to stretch bass strings.
I don’t know that much about this stuff, but I kinda think if they wanted you to pre-stretch the strings, they’d do it at the factory. Again though, I’m completely oblivious to this subject. 
What in the world is the point of stretching the strings? That strikes me as a colossal waste of time.
I pre-stretch, I guess because a video I watched with Jason DeSalvo from Fodera talked about doing it in his bass care video series, although he was using their strings and not LaBellas. Bass Care & Maintenance: Changing Strings, Oiling Your Fingerboard, and Final Adjustments - YouTube
Guess I will not pre-stretch my LaBellas (which are my flats of choice) in the future though. Thanks, @John_E.
At least on guitar, strings (esp unwound) stretch significantly when they’re new so if you don’t stretch them, they’ll continue to go flat until they stretch out. You can either just accept that or you can spend a few mins stretching them and then you’re done. On bass, I haven’t found that to be necessary.
They’ll go flat and you’ll need to retune several times. Which may not be a big deal noodling around at home, but you don’t want that at a live gig.
I saw the Grateful Dead once where Gerry snapped a string, and I remember that show as Tuning with Gerry as he retuned after every song the rest of the show. Stretching is standard practice
Should add this was in the days before tuners and he had to tune the strings to each other constantly
I pre-stretch & tune higher than I need to with nylon strings for my acoustic guitar only. That’s just to help them stay in tune. Otherwise I never pre-stretch with any other strings.
At first I thought you meant tuning machines and it though “man, that IS a long time ago” 
I’ve changed dozens of sets of bass strings without pre stretching ( never heard of it until now) and none have ever failed me in any way.
I’m going to carry on with my rebellious ways and continue NOT pre stretching.
You go girl!
@Wombat-metal It was not before tuners, tuners have been out for close to 80 years Mr. Garcia must have just been cheap!