Okay I always recommend to use own domain. As you own it and you will get lot more value from your customers. Now if you use co.cc or dot.tk or may be cz.cc,I dont know much about them but once i used dot.tk and host that blog in blogger.But after 3 month dot tk delete the domain i registered with them.So if you want to use it commercially then buy a domain would be best.
Ifyou are learning basics of websites ,how to host your site and any other webtool then you can use free domains for that .but don't use it for business. MOst important thing is .co.cc is not free for business websites from 2nd year .it will charge you 3$ per domain .so its better to use coupons and get a .com for 6.99$ .