Working out equivalent resistance
Yes generally speaking using an indipendent voltage generator as a stimulus to find equivalent resistance is more than correct, it is applying its definition.

Just a few add-ons:

It doesen't have to be a voltage generator, can be either voltage or current, very often one of the two choices gives much simpler maths.
So before starting reckless calculations it is definitevely worthwile having a short survey to find out which is best in that case.

When we say parallel resistance being \$R_\text{p}=R_1R_2/(R_1+R_2)\$ we have actually connected a voltage generator, done the maths and finally remember the result by heart.
This is usefull since it's such a common problem, it's no use re-invent the wheel every time.  

The same applies to series resistance (but with a current generator though).

A few other configurations may be worthly learned by heart, e.g. common collector basic output resitance is \$1/g_\text{m}\$ parallel something else, basic BJT input resitance with emitter degeneration is \$(1+g_\text{m}r_\pi) R_\text{E}\$ series something else, and so on depending or what you need/ are studying.

Recapping, use equivalent resistance defintion, hence voltage or current stimulus, is always correct but usually longer, so should be reserved to cases not covered above.