Do you believe you are experiencing this, or are you just trying to plan for it?
There are a lot of inaccurate statements you can find in blog posts about EC2.
The alleged "noisy neighbor" problem is a frequent topic, and drowning in inaccuracies.
Sure, dedicated instances would solve this problem -- if it were a problem -- by giving you a new problem, as has been noted -- you'd be your own noisy neighbor. It would make no sense for AWS not to cluster all of your dedicated instances on any more physical servers than absolutely necessary.
But, neither one is the case, because it's an imaginary problem caused by a widespread lack of understanding about what "stolen" CPU cycles are, and what they mean in EC2.
The EC2 infrastructure has a number of different physical configurations. Even within the same instance class, depending on which type you're deployed on, and your own load, you'll see more or fewer stolen cycles, for a reason entirely unrelated to "noisy neighbors."
The "m1.small" instance class frequently shows 50% of your CPU being "stolen," because they've provisioned you on a physical CPU twice as fast as what they're charging you for. The "t1.micro" instance class throttles you down to 10% utilization quite predictably after about 15 seconds of 100% CPU utilization, and doesn't open things back up for two to three minutes -- and it's very predictable and repeatable, not random -- and, yes, this throttling is done by "stealing" cycles.
Eric Hammond clarifies this quite well:
Depending on the EC2 instance type and the underlying hardware, you may not be paying for access to all of the underlying CPU cycles. Amazon is not going to give you access to 100% of a modern, fast CPU if you have asked for an m1.small which is promised to be equivalent to an old, slow CPU.
On EC2, steal doesn't depend on the activity of other virtual machine neighbors. It is simply a matter of EC2 making sure you are not getting more CPU cycles than you are paying for.
— High CPU% stolen on EC2 instance at extremely regular intervals (serverfault.com)
I've certainly over-subscribed virtual machines to CPU resources on physical hosts running multiple VMs in my own data center, and there are likely some virtual hosting providers that do this, so I'm not saying it's not possible anywhere or doesn't happen in any virtual environment... I'm only saying if you think you are having this problem in EC2... look for a different problem.
The primary purpose of dedicated tenancy in EC2 seems to be "so you can say you aren't running on shared hardware" for regulatory or compliance purposes.