passport stamp in porto

You can't. The only thing you need worry about is your entry date, and verifying that any entry or exit from the Schengen area is documented, either by stamp, or you retaining proof of entry or exit (plane tickets, hotel receipts, etc). Soon, maybe by the end of the year, it will all be electronic...no stamps at all.

I read somewhere that if I have my passport stamped when entering portugal it will be easier when I leave.

Since that completely negates the purpose of the Schengen Agreement, you either misunderstood what you read, or you read this more than 35 years ago. Or you read nonsense from someone that didn't know what they were talking about.

When you legally enter the Schengen area, regardless of the specific country, you are legally allowed free movement anywhere within it. And you can freely exit from any exit point in the Schengen area, regardless of your point of entry.

What problems do you anticipate when leaving?