GH, I was trimming my mares for while. I did lots of learning, DVDs, reading, and asked for the help in very beginning (later on I also took a farrier class in UMD). The reason behind: they were unhandled, hard to deal, and with market here farriers were not interested in dealing with problem horses.
Was it better than no trim at all (given one of them NEVER was trimmed when I got her)? Heck, yes. Would I advise it? Depends. I think it's OK to alternate if you know what you are doing and horses are not in hard work AND neither horse has funny hoof conformation that require a very good knowledge/experience. I personally found it to be hard to do balancing correctly (everything else was rather easy). Now when I ride them quite intense (plus have some other issues) I prefer knowledgeable farrier to do the trimming.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."