da Rock

Actually, that depends on how tail heavy, and it also matters what you call "tail heavy".

It's always safer to balance on or ahead of the suggested start CG location, and especially if you use a "rule of thumb" to guess where the CG might be.

However, the plane won't become uncontrollable until the CG is pretty far aft of a recommended location. (Recommended by the designer or a reliable CG locator like geistware.com)

It appears that an awful lot of modelers have decided that "tail heavy" is an instant death warrant. It'd serve beginners better if those dire sound bite warnings had a bit of explanation added. Because CG locating and location aren't sound bit simple, but aren't hard to understand either.