No, recommended practice is not a full measure "empty", it's a measure empty of music notes
so that all the necessary setup messages, such as these SysExes, can be inserted here.
There are many very useful files authored by/for Yamaha XG demonstration (and for Roland GS) that warrent analysis at the List View level.
(Actually, it's how I learnt much of what I know about MIDI programming
They simply set up with a GM system on at 0001:01:000 (at 480 TPQN) then (most often) the XG on occurs at 0001:02:000.
(Leaving that much time for the GM on is an overkill in my view
All the subsequent setup messages the occur from 0001:02:120 onwards.
See the attached setup picture with the master track at the top and the first instrument track below.
The calculation is straightforward.
At 480 TPQN 240 ticks are 250 milliseconds. Very
approximately 1 tick per millisecond.
At 120 bpm there are two beats per second in an X/4 measure, e.g. 4/4. (So a 4/4 measure takes two seconds to complete.)
Therefore 1 beat of 4/4 at 120 BPM is 500 milliseconds long.
Halve the BPM to 60 and each tick is twice as long, double the BPM and each tick is roughly half a miilisecond.
Look around for some examples of XG files authored by or for Yamaha and you might be amazed what you can learn from them.
Just my 0.02c worth.
As I read it it's 100ms for the GM reset, which has to reset pretty well all controllers back to their default state.
Then 50ms for for an XG or a GS reset.
Why are they needed?
Because in a hardware sound module the processor has to execaute a whole series of instructions doing the reset for (in the case of e.g. an MU1000) 64 channels in total.