When traffic shaping is applied on the MR, is this applied to the traffic on the wired side or the wireless side (over the air)?
I ask as surely it would be bad practice to rate limit traffic over the air? Every wireless client should transmit and receive data at the highest rate possible to reduce their contention on the air space.
Is it therefore best practice to apply traffic shaping on the MX?
Traffic shaping, on any device or interface type, from any vendor, does not slow the phy rate that data is sent at. Shaping is based on a token bucket model that allows a given amount of traffic to be sent every time period. Once that device's allotted data has been sent for a given time period further traffic is held back and not sent until the next time period begins. Typically these time periods are in the milli- or micro-second ranges, which give the perception in Human understandable time periods that traffic is "slowed".
A gigabit link between two devices can only send traffic at gigabit speed. It it not capable of sending at different speeds. What you observe as "shaped" traffic is data being sent for a couple micro-seconds, and then a couple micro-seconds where no data is sent, repeated over and over.
A wireless client will always send at the rate its connected at (MCS), but if traffic was shaped the AP would simply just not allow it as much airtime, which translate to less chances to send data.
To your question though, I don't know what the answer actually is on a Meraki MR, and it is a good question. My assumption is that the shaping is enforced egress on the Ethernet port, but I do not have any data to support that position.