For species identification is more than OK. If you want to characterize sub-species or other more related groups you must complement ITS1 with other sequences or biochemical tests. See an example of identification of a Penicillium strain by ITS sequencing.
ITS is typically the barecode for fungi, but it has some conflicting regions for identification. Some authors recommend to combine ITS with a codifying sequence as EF-1alpha or RPB1. Look for what is the better option for the phyllum of your fungus that you're trying to determine.
If you do not want to be strict, a search of your ITS in BLAST will be enough.