Tipping practices can vary depending on the country and the service industry norms, so it's essential to consider the local customs. In the United States, tipping is generally expected in the service industry, including for massage therapists. While there is no strict rule for how much to tip a massage therapist, the customary range is usually around 15% to 20% of the massage cost.
In your situation, a $5 tip on a $50 massage amounts to 10%, which is below the customary range. The receptionist might have considered it rude because it was lower than what is typically expected. While tipping is ultimately a personal decision, providing a tip that aligns with the customary range is a way to show appreciation for the service received.
If you are unsure about tipping practices in a particular establishment or country, you can always ask the staff or do a quick online search to determine the appropriate tipping etiquette. Remember that tipping is meant to be a gesture of gratitude for good service, so it's considerate to tip appropriately if you were satisfied with the massage.