Since the web is based on HTML, it doesn't really matter which program you use since HTML is HTML.
Ok, some programs (most notably Frontpage) add a lot of unnecessary code that may cause some compatibility issues but nothing too serious.
You should have no problem switiching between Kompozer and DW. Until I settled upon my program of choice (Coda, Mac-only) i tested Dreamweaver, Kompozer, BBEdit, Notepad, iWeb and a whole host of others.
My advice is to thoroughly learn HTML and then you can be sure that the code you see on the screen produces the result you require. You will then be able to build your sites (including CSS) in the most basic of text editors without ever having to purchase software...although Kompozer is free so is a good option anyway