we are planning a trip to Phuket at the beginning of Oct. Is this not a good time to go?

No, not if you want to avoid the monsoons.

Take a look at www.weatherbase.com

October is a difficult month, both sides of the peninsula have a lot of rain then.

http://thailandforvisitors.com/south...t-weather.html

October is a month with quite a lot of rain, especially the beginning of the month. It wouldn't be unusual to have a mostly sunny day with afternoon showers though.

Is it a good time to go? Prices are very low, so you can find some really good deals.

Weather might not be so great. If your enjoyment relies on sunny days and clear seas then maybe not a good time. Seas can be very rough and there are more dangerous currents for swimmers at this time of year. Visibility may not be good for diving.

I have just put to the top under 'rainfall may-Oct' but have added Nov thru Jan also.

Phuket Sept is 16.5 and Oct 12.0, it 'is' the rainy season, it depends how much the rain will affect how you want to spend your time. Hotel rates are 'right down' at that time so you can get bargains.
The main season is Nov thru mid April because of the good weather.

Hotel prices are usually at least half of high season rates at that time. Another advantage is that the weather is cooler.

It does depend on what you would like to do. Personally I don't mind (actually prefer) sitting on the beach on an overcast day.

Service is also much better as places are not so crowded.

You can check on the lower prices at:

http://phuketdelight.com/phuket_hotels.htm