The Teensy has an internal regulator that makes 3.3V from 4-5.5V on Vin, which is normally connected to VUSB. The Pi is much the same, stepping down to lower voltages with a bit of tolerance on the 5V. Where things start to matter is not at the Teensy but the power supply/s. If you have a single stable 5V power supply doing USB power to Pi, Teeny and LEDs then things will be fine. Problems start to arise if the LED power gets ripples and noise from the LED PWM or other sources and this disrupts the Pi or Teensy CPUs or where multiple different power supplies are connected via the various 5V USB connections.
One problem can be that the low current supply powering the Pi is actually 5.1 volts, while the big supply for the LEDs is set for 5.0. Current flows from the 5.1 to 5.0 power supplies effectively shorting the 5.1V supply. Solution here is to ensure that all grounds are shared, but the LED 5V is not cross connected to the USB 5V at any point.
Generally any solution that has power supplies connected to each other is a problem, unless the power supplies are identical units and designed to share loads.
If situation requires you can supply the Teensy through the 3.3V input rather than having the onboard regulator make it from 5V, but it is really critical that this is stable and never rises beyond 3.6V or it will kill the Teensy very dead.
