A long time ago, I used to work at a company called XVT Software, whose main product during that time was a C-language-based user interface portability toolkit. (Yes, we had other products, but the C toolkits were the bread-and-butter). For those of us on the development team, the mantra was 'port early, port often.' We had to deal with lots and lots of environments, and as anyone in the biz back then knows, even within (or especially within!) the Unix-like family tree, the differences between vendor implementations were huge. So you saved yourself a lot of trouble by following our mantra.
Even though as a complete Lisp newbie I knew there were differences between CL implementations, I have only recently started to understand at a deep level how much our old mantra from XVT applies to the CL world. I now have code running well on LispWorks, CLISP, and SBCL. And what I've discovered is that code porting, rather than being a negative, is actually very healthy. In my experience, it has been a process of fine-tuning and improvement, as each new compiler points out bad habits, opportunities for cleanup, etc. In fact, I would go so far as to recommend folks with single-vendor codebases to make some effort at porting to at least one other implementation -- no matter how good your current compiler is (or how good you *think* it is), there will almost always be new/useful tidbits that another compiler will point out. If you try it, I think you'll be glad that you did.