The Sigma 150, 180 and the new Tamron 60 f/2 don't extend when focusing. The Tamron is cropped sensor only. I think the Tamron 180 maintains its length as well but I'm not sure.
I don't think the extension would be a problem. For example if the lens has a working distance of 10cm, and you get as close as you can before acquiring focus, focusing to 1:1 will extend the lens but it should never hit your subject. The only problem is if you're taking a picture of something by shooting through some other obstacles like a fence or leaves.
My Nikon AF-S 60 does not extend but has one of smallest working distances of any macro. It helps that the length is predictable and constant but I've never had any issues hitting anything, so I don't think any of the extending lenses will either. I don't shoot bugs so often though, and the big lens moving in on them might change things.