I think the sizing depends largely on the manufacturer. I recently ordered a Pinnacle Inferno V-Skin (which is similar to the Sharkskin and Lavacore skins) and I was going to drop down one size from my wetsuit (and that was based on actual measurements). I normally wear a 2XL Akona Quantum stretch full wetsuit. I attempted to try on an XL Pinnacle V-Skin and it wasn't happening. I was never going to get into it. So, I ended up ordering a 2XL Pinnacle. Still waiting for it to come in from New Zealand so, we'll see for sure when it gets here.
All that being said, it is a good idea to order one that's a little snug. However, you'll never know for sure what "snug" is unless you actually try one on. It may be a pain to get to a big dive shop to try on several different sizes but, I promise you it will be worth it in the long run.