Well, the is extra expense when you take the head off because you have to buy a new head gasket.
There are a couple of ways to pull the valve springs off when the head is on the car. Either way, you're going to need a special tool that will allow you to compress the valve springs while the head is on the car. The only company I know of that makes this type of tool is Snap On. There might be others, but I am not aware of them. Sun also told me that he had to pull the hood off (to get enough clearance for the tool), but others have told me that they didn't have to do it. I personally have not done it, so I can't say what you would have to do. I would think the major problem is finding the tool, as it's not cheap ($200, IIRC) and most shops don't have them for rent. You will also need either a compressed air source (i.e. compressor) or some medium sized clean rope. The cleanest method I know is to use an adapter to adapt the air hose to your spark plug holes. Use the air compressor to compress the air in the combustion chamber to hold the valves in place (of course the engine needs to be at TDC on the compression stroke for that particular piston). Then remove the valve springs, retainers, and retainer clips. The other method, which is more of a backyard mechanic method is to put the piston down to BDC. Thread some rope into the combustion chamber until it feels almost full. Turn the engine over by hand until you feel resistance. The rope will compress and hold the valves into place.
The main problem I found is that installing the keepers can be a real PITA. That's with the head off the car. I couldn't imagine doing it with the head on the car and the hood in place.
Pulling the springs with the head off the car is easy. I changed a full set of valve springs, retainers, keepers, seats, and seals within a couple of hours.
If you decide you want to pull the head, I have a valve spring compressor you can "rent" from me.
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