We are using an idea like this at my shop. We are about 10 people and I run the tech, if we were bigger it probably would not work. I don't lock boxes down. We let people who want to pick out what they work with, you want a desktop with 3 monitors? Sure. A 17" laptop? Sure. A great travel 15" laptop, fine. Mac Pro? No problem. And when asked, "Can I install X, Y, Z on it?" Sure, it's yours basically, do whatever you'd like with it. And when you have problems we'll try and help you fix YOUR machine, while we are blowing it away and resetting it up YOU can use this old machine for 2 days. It's a tool to do your job, do what you want with it.
Basically I've found that if you have them think of that machine as theirs, they treat it better. So instead of their mindset being, "Well, I'll just download this, my antivirus will stop me if it's bad and IT can deal with it" They have been thinking, "It's probably not worth messing with, I don't want to screw up MY computer."
Like I said, we are willing to help them, but I tell them straight up that it is their laptop/desktop to use. When some people leave we've just let them take their old laptop with them. I mean, they picked it out, it's got the stuff they want on it, and it's 2 years old, take it. The new guy will want to pick his own stuff out.
If we grew much bigger though we'd need more in the line of policy.