Is it a good practice for a newly created company to offer free services to the very first clients?

Is it okay, yeah. Would I personally? No.

Your time and talent are worth something. Never devalue yourself. Once you set that precedence, it will be hard to revert from it. (This goes back to how you position your business/your brand in the market, it's all about perception.)

Businesses and individuals alike all charge for the service you mentioned above. Even if you're just building your business, you should still charge something (based on your experience level & raise your rate as you gain more experience/clientele.)

If it is sampling for a portfolio, perhaps consider doing work for a startup in exchange for some equity & a testimony. Or a charitable cause for a testimony. You could also do spec work (build some sites to show as examples of what you can do) and sell them off later.

Is it ok? Yes. You won't go to jail for it.

Would I do it? Probably not, no.

If a client doesn't value you and the solution you're building enough to want to pay you, then you're either building the wrong solution or working with the wrong kind of clients.

I don't really see why not. It's something I am doing with my SaaS although I don't advertise it on the website itself.

I mention it in posts and comments that if users would like to reach out to me and I can add time to their account.

Now specifically for your situation, I have built websites for free in return for building my portfolio in the past - but that was very early on and it did help me land a job at the time, and sure you also get experience too.