From the same question a couple of weeks back:

It works fine as long as there were no symptoms of contamination - molds, fungi, etc. in the previous use.
Some advocate dousing it with boiling water before re-use.

You may be surprised when the seeds that did not germinate, do so when the mix is used the next time. Not usually a problem, if you are not surprised. Al

Should be okay- I do it all the time! I sometimes get little 'surprises' but that's part of the fun.