Sim, I keep leftovers from my exposure checks and cut those up to use as toning test strips. Unless you make fresh toner each and every time, the time response and character will change, and so it's useful to have test strips to see what would happen if you went further in the toner. Of course, you just pull the strips out at different times and after quick drying (microwave!) see what you'll be getting.
Do be sure to have done a complete fix and wash before toning, of course. This is something that needs a bit of extra care with the fiber based papers. If you see radical changes of colour relative to what you had before, you might ask yourself if there was any difference in your fix/wash procedure, e.g. if the paper was as thoroughly pre-wetted as before and so forth.