Not true, certainly not for most cases. What makes you think so?
What false alarm are we talking about in particular? (malware name, file)
There's a list of exclusions for the Standard Shield - you can use it.
I think it was the Photokit plugins - took over a month before avast! didn't pick it up as a Trojan - to be honest, I didn't submit it, just posted in this forum, so I assume a Mod did it.
I contacted Photokit about it at the same time as I mentioned it here though, and their mail said that they had already contacted avast! about it?!
Anyway, no biggy, just thought it would be nice if there was an Ignore function.