I agree with Webber. Although Google still makes plenty of mistakes, they are in tune with the issues of multiple sites at a single IP address. Although Google still records information by IP Address, I believe that they use TLD as a key, and not IP, these days, especially since people move to different hosting systems, more than they change Email Addresses. IMO, the cost for paying for a dedicated IP address, may not be worth the extra cash.
If you are still concerned, another laborious task you can do, to check things out first, is to ask your hosting company to tell you which Class C block they are planning on putting you under (in the shared environment). You can then TRACEROUTE or BROWSE each of the 255 potential IP addresses under that class C, to see if there are any “shady” sites there. Of course, this method will not reveal any “past” sites that were hosted under the Class C, and will only reveal the current state of affairs.