It doesn't really matter if you knock off a few blooms (although there's no reason why that should happen. They're not particularly delicate). Lantanas repeat bloom over a long period. They don't just have one set of flowers.

Was just concerned about heavy rain knocking blossoms off

Lantana is no more likely, in fact even less likely, to be damaged by heavy rain than any other plants you might use.

When lantana inflorescences are past maturity, they do easily shatter or fall apart. That’s completely normal. Until that point, they are pretty sturdy, as floral says..

I've never noticed that problem with lantanas -- as noted above, the flowers are sturdy.