I'd use a high potassium tomato-type feed - the potassium is meant to encourage flowering, and I tend to feed just while they are in growth. I see the logic of what your nurseryman advised but would worry about overdosing them then and scorching the roots. If using artificial fertilisers regularly you can get a build up of salts and it is worth giving a thorough soak with rain water from time to time to flush them through, particularly if you don't re-pot annually.