vagulus:
Thanks for that, ElCaron, but I am not quite with you.
vagulus:
No. If I have this the right way round, Boxall uses pullup on the lEDs and pulldown on the switches. I use pulldown all over.
Sorry, I have only looked at the switches. My answer also essentially just references switch applications.
vagulus:
Am I to understand that each output pin on my UNO has a built-in pullup resistor?
Yes, but it will only be strong enough to switch logic with trace capacitance that you expect on a PCB and only if it is not too fast. It will not be strong enough to provide enough current for an LED.
I don't understand what you are saying here.
When you design a PCB, you connect one switch pin to the Arduino. The other side needs to go either to GND or to VCC. GND tends to be ALL OVER THE PLACE, so it is not hard to connect to it. VCC may be far away.
The pull resistor "direction" depends on what you choose. It will be in the area of the MCU, where VCC is certainly available, or even built in. So in general, routing a PCB will often be easier if you choose a pullup for a switch.
For the LED, it really depends on the situation. In most cases, LEDs with pull resistors are a secondary indicator for another circuit. (Like TX/RX on the Arduino). The the choice entirely depends on when you want the LED to light up.