I know people who do maintainance trimming inbetween visits. There is legaly nothing to stop you so long as you are not trimming in preparation for a shoe.
If you do some road work then Mother Nature does the best trimming and she knows what she is doing rather than owners!! I lead mine in hand round the village twice a week to keep their feet trimmed and my farrier who does my other horses with shoes tells me they are the best trimmed feet on his rounds!!
Do you know Dressage_Babe, I always thought that, but these EP trimmers say that is not so and the hooves MUST be properly trimmed. I do know a few people who do minor rasps to neaten up hooves between farrier visits, but would not even do that myself, it is far too easy to ruin balance etc.
My farrier is a superb remedial farrier who has been flown over UK sorting out top SJ & DR horses he was also farrier to Mary King for a while, I would never doubt his work or his word, he sorted a youngster out that I had with side bone & navicular changes just by damn good farriery!! If he tells me Mother Nature is the best farrier going I am not going to doubt that! all my horses have no back shoes and they hack 4 hrs a week with solid horn, and huge cushioning frogs due to Mother Nature apparently!! this is his quotation 'If Mother Nature became a Farrier we Farriers would all be out of business'