Whenever IMO. I clipped Flo in May last year and she looked fab come July. Some people say February/March time but I really don't think it matters unless you're planning on showing that is. Also some horses are clipped all year round.
I was told by somebody who shows and breeds showponies very sucessfully, that Valentines day is the latest you should clip to avoid disrupting the early summer coat.
I clip Inky all summer from March to September as miniatures are shown clipped. Ive never had a problem with his coat and never got clip lines as you can see. He was clipped two weeks before this pic.
I was always told February, I am going to full clip Mr T even though he not really doing much work as last year, unclipped he still had some of his winter coat into June and started to grow a new winter coat in August, flippin weather. Logic was that if he is full clipped he wont have ugly clip lines as I was to show him and can't predict the weather.
You can clip whenever you want to clip...I used to clip my Welshy out most of the year. The last "winter" clip I did on him was end of Feb/early March and he always got a lovely summer coat then (which I promptly clipped off if he got too hot!).
Many say not to clip after February as it supposedly causes problems with the summer coat coming through if you clip later? There again, many people clip all year round without problem. I think that if you have a partticularlky hairy horse that sweats alot under work then clip him as & when.
If I didn't have a pink pony that burns easily and was also not a nightmare to clip I probably would do it all year round, but he does get burnt a bit where he is hogged : ( so do not want to put the rest of his body through that!