I'm late on getting a website up for a local doctor. Since she wants to use a CMS anyway, I was thinking about using a template that comes with the CMS WordPress. Would doing this be considered "cheating?"
As long as you are fulfilling a proposal agreed with the client and are sure their expectations will be satisfied by use of the technology you choose to use, it's unlikely that accusations of "cheating" would arise.
If you are in doubt, or are veering from an agreed spec, discuss and clarify with the client.
It's not uncommon to do that. But as said, make sure it doesn't go against any agreement you made. A template made by someone else can be harder to work with / modify, so make sure you don't take on more than you can handle.
As long as the client is okay with it then it should be fine. If they paid for a unique template and it said in the contract that it was going to be a unique template then you may have an issue. Legally, the contract rules.
There is nothing wrong with CMS templates, I've bought a dozen or so of them and will continue to do so. Now, if a client wishes to pay me for my time, I am more than ears to designing and coding their website. It's all to do with budget. In fact, I tried to make custom websites more attractive by lowering the price, but miraculously people, even big international companies, seam to want templates, not sure if this is a regional thing, but it seams to be the message back home. You have to let them know it's a template, don't sell template work as custom work!.
I would never change the template on my own, I always try to stick to how it was made, as @ralph.m ; mentioned, it's often harder to work with a template in terms of editing, especially now with CSS3, box-shadow this, border-bottom that, text-shadow that etc. Imagine a changing all those colours, simple changing the navigation bar colour would involve changing 4 - 6 properties, so say the least.
Purchase your template with more than one colour variation, and make as little changes as possible to the template. Preferably, remove the unwanted sections and select the colour scheme desired.
To answer you question simply, if you don't tell them, then yes you're cheating, and could possible face legal action if you sign a contract on custom work. If you did tell them, then you'd have no issues. I always give two proposals, one being a template and another for tailored. I also sell stock logo's, but with the mention of telling them. For the logo to be custom it would be x3 the cost, likewise with a tailored solution website, and that's my being cheap, let the clients know of this and I am sure you won't have any issues.