Getting cards PSA graded is very situational.
I would only suggest sending in cards that you are 100% sure will receive an 8, 9, or 10. On top of that, don't just send in any card because the grading process is expensive. The regular price is $15/card and you need to pay for shipping both ways. That's easily another $30. Personally, I only send in cards that are old, expensive, and Mint condition or better. I usually only send in cards that I believe will receive a 10; however, on rare occasion I may send in an extremely expensive card if I believe it will get at least a 9. That doesn't mean it will, but that's how I judge whether or not it is worth it. Graded cards can be VERY valuable depending on its age and rarity. Even newer cards can be worth grading but they usually don't gain too much value immediately after the process.