Of course it is. You'll find whole different version of Spiti around Oct and believe me it's the one that very few prefers. Probably it could start snowing so plan your trip accordingly.
We went to Spiti in May end. It was beautiful - from getting snows on the sides of the road to snow peaks, even all the roads get cleared. So, October is a different beauty for sure but if you go during this time, you can travel to more places with less risk factor.
Plan in early October
It starts to snow at Kunzum top sometime by mid-October which means that if you were traveling within first 10 days, you will be able to complete the Spiti circuit. Chandratal too remains open till around 10th so that too can be included in the itinerary in early October.