Logic minimization guarantees of arbitrary subcircuits?
As a rule yes, if it's truly minimized, sub-blocks will also be minimized.

However, minimizing logic is sometimes not the same thing as minimizing devices or gates. 

In some cases you can see odd logic configurations which are done solely to reduce the parts count by making use of unused gates in existing devices. By using spare gates rather than adding new device(s) you can sometimes reduce the parts count and cost.

For example:







simulate this circuit &ndash; Schematic created using CircuitLab

Notice the second circuit would use three different logic device types, whereas the first uses a single chip. Logically they are the same.