Does My Piano Need Tuning?
If it's been over 6 months to a year since your last tuning, the answer is, yes. (In my experience, no more than one in 100 pianos can hold a decent tune longer than one year.)
When a piano is fully tuned up to standard pitch (A440), there is approximately 40,000 pounds of tension on the strings. As the piano sits, this pressure works to slowly cause the strings to slacken and the piano gradually goes flat.