I think it very much depends on your conscience. I think if you were able to alter a theme to a degree far enough that it resembled what the client outlines in their brief then sure, go for it.
Maybe download a few themes set up a development environment with MAMP or WAMP if your on windows, or buy a domain and some hosting and get your hands dirty with the code, make some mistakes, scratch your head and get to the stage you want to smash up your computer. Then you will be getting somewhere!
I think themes are good to rip apart and learn from, but they seldom offer everything you or a client needs. I expect there are a few, infact I know there a re probably quite a few developers out there that fob off their clients with these templates, but think about it, is that really what YOU want to offer?
Just some food for thought, good luck with what ever you do
I was in a very similar position to yourself a short time ago — having to design and implement websites for friends, family and small organisations. Whilst the advantages are that the work has all been done for you, and some are easy to make minor alterations to, I found that anything more than merely cosmetic changes was beyond my skills.
I had a good enough knowledge of HTML — back in 1997 — and ended up bringing myself up to speed with HTML5 and CSS and worked my butt off for a week, cursing IE6 daily. Eventually, I was able to put some sites together myself, and more importantly, I knew how to make alterations to it; something you’ll no doubt be required to do if you went into business with it.
I agree with Marcus in that taking apart prewritten code is a great idea, but unless you know what the code is AFTER you’ve viewed the source, it’s not really going to help.
I have used those types of templates simply to look for new ideas and learn how to do new things. It is a good way to learn. Now, I always make all of my stuff from scratch. If you use those free templates without understanding how they work, I guarantee at some point your customers is going to ask you to alter something in it. That’s where the problem starts. If you don’t understand how each component works, it can be very stressful to have to modify it.
As you do more projects, you’ll develop your own library of work. I often copy things out of one of my projects and use it to jumpstart another in order to save time and work. It’s all based on things that I understand because I made the original product.