Since I started working with micros I've always added an RC on the reset pin. This was always called for back in the day. But now I'm thinking I missed a memo and that RC is not necessary for AVRs (e.g. Mega328, Tiny84 as examples). I just looked up an Arduino schematic and they don't have a capacitor on reset, just a 10k R to Vcc. So, is that what I should be doing, R (which I understand the need for) but no C?
valusoft 
The only time that I have used a C on the reset pin is if I need to hold the chip in reset for a little longer than "normal" to allow other devices on the same power supply to stabilise. YMMV.
It is supposedly a good idea if your AVR will live in a high-noise environment. Maybe you are gonna make an arc-welder, so a cap won't hurt.
gchapman 
So, is that what I should be doing, R (which I understand the need for) but no C?
If reset is close to a ground plane then no capacitor.
If reset is on a connector then at least a capacitor though more complete ESD protection is recommended (R-TVS-R though R-C-R may be good enough [reduce edge speed, current division])
Connection of RESET Pin on AVR Devices | AVR® Microcontroller Hardware Design Considerations
[end of fourth paragraph]
However, such extra capacitor cannot be used when DebugWIRE or PDI is used.
Reset Pin Protection | AVR040: EMC Design Considerations
[end of second paragraph]
In addition, a pull-up resistor (10kΩ typical) and a small filter capacitor (4.7nF) should be connected as shown in the figure below.
wrt reset, an AVRISP mkII has significant drive whereas an Atmel-ICE doesn't.
The Art of Electronics 3rd Edition | by Horowitz and Hill
[page 16, middle of right column]
12.1.5 Input protection 804
js 
Since I started working with micros I've always added an RC on the reset pin.
+1 however the cap on the pin will stop DW from working. So the board I use for debug has no cap.
The old uC needed a reset IC to prevent malfunctions when the power supply started slowly.
RC was a simple alternative.
AVR has a built-in POR. There is also BOD.
Now C is just a designer's tranquilizer.
It is supposedly a good idea if your AVR will live in a high-noise environment. Maybe you are gonna make an arc-welder, so a cap won't hurt.
Funny that you say that. I worked on this machine: https://youtu.be/tpzZaTTCCoc?t=214
