All FIXED DECIMAL variables that you use must be used only when they contain valid data.
If a FIXED DECIMAL variable contains invalid data (such as bad numeric digits or a bad sign nibble), any use of that variable may lead to a data exception. Even the assignment of such a variable to another variable with a similar precision and scale may lead to a data exception - even though the assignment could be done via a byte move.
Conversely, you should not presume that a data exception will be raised on the first use of such a variable: for example, the assignment described above may be done with a byte move under some circumstances, and in that case, a data exception would not occur until it was used in an arithmetic operation or a compare etc.