We usually take our trip in Feburary or March. We have been during other times of the year but we like Feb/March better. It is usually cool, some jacket days, and sometimes it will be shorts weather. I live in the south so it is a good time of year for me. It hasn't rained on us much during that time of year, but we carry ponchos anyway.
I haven't noticed the lines to be any worse during this time of year than any other. My DH and DS(18) loves SM at MK and they usually can walk right on and ride several times in a row. The only lines I have really encountered are not that long ( waits usually 25-30 minutes) . You can usally deduct 10 minutes from the wait times, I have found.
I hope this helps.