Well,you just have to look at the different seasons in Tibet to really ascertain which will be worthwhile and more comfortable for you. People like to travel at different times but you just have go with the best for you.
The spring is often between April and May and during this period the weather is very warm and tourist can find this period very convenient.
Sunmer time is between June-August and this is like the busiest period for tourism. The weather here is characterised by warmness, a bit of cold and rain too.
Fall comes in September through October and everywhere is just cool and almost like the best time for tourist where one can go hiking and trekking , there's less cloud too.
Winter set in from November-March with alot of coldness and snow which I believe isnt a great time for tourist.
So by and large May-october could just be the appropriate time to choose from.