Somewhere I saw a diagram where the Arduino Mega was being powered through its Vin. The diagram showed a 220 ohm resistor attached to the wire to limit the CURRENT and -- I think -- the other end of the resistor went to GND. I have not been able to find that webpage again, but it stated that this was a good (recommended) practice.
The space where the Mega will fit in my project does not allow for a USB B plug or a wall wart's power plug. I can however use a PC's USB A port as the source for +5v and GND. So I want to wire those to Vin and GND, or to 5v and GND.
My 1st question is "If the voltage is between 6(or 7) and 10 volts (recommended range) then does it really matter what the current is?" (I don't know how to calculate it).
My 2nd question is "Since the PC's USB A's pin 1 emits +5V (much less than what Vin expects, can I instead wire it into 5v "pin port" on the Mega? and will that need a current limiting resistor?".
No, you don't need a resistor in series with the power. The current is determined by the effective resistance of the Arduino itself... For example, if you have a couple of LEDs connected and your sketch turns them on, a little more current will flow to light the LEDs.
...Someone may attempt to use a resistor to drop the voltage if a higher-voltage power supply is used. That takes some stress (and heat) off the built-in voltage regulator. But, that's generally a bad design because the voltage drop across the resistor depends on how much current the Arduino is drawing at the particular moment.
You do need a resistor to limit the current through an LED because LEDs are non-linear and they actually run off about 2V. If you apply 5V, about 3V is dropped across the resistor and the resistor determines/limits the current. And since the resistor & LED are in series, the same current must flow through both.