I've heard it as well, but don't think there's much truth to it.
Copper does work harden, so in theory, if you bend it enough it will eventually break. That has more to do with the wire than a soldered connection though.
What really matters is using shrink tubing, or something to support the connection, so that it's not hanging by the copper.
I prefer crimps just because they're faster and easier, but solder works just as well. I buy bulk shrinkable crimp sleeves on ebay, way cheaper than anywhere else.