Well, its usually better to use new soil....

But if you think about it the new potting soil you buy, is also "used soil" in some way....it just has been enriched whit stuff....

I personally have used old soil from my prev. grows without any problems, but mixed it whit other stuff(potting soil, perlite,etc)....
In my case the soil doesn't need to have any nutrients, i feed my plants whit liquid nutes....the soil is just a "neutral" medium.....