Clients love onsite time, yes. It makes them feel appreciated and well serviced. But, it's usually not nearly as useful as it sounds. And, as clients become more experienced they'll be less interested in spedning precious hours having you onsite. So, the bigger the client (and better) is the LESS onsite team you'll need.
I have done well with a little rule of thumb that I use. Come up with visit per dollars approach. For example, if you are just starting out and you are hungry, maybe allow a site visit for each $1000 of revenue. After you climb the client ladder and have more sophisticated projects with more mature clients, maybe one site visit per 10k, etc.
In the end, site visits are usually for 'feel god' and not for productivity, but each client is different.