When are ATC frequencies preset in the radios?
I'm not aware of any rules, hopefully others can contribute, but you can usually predict the next frequency with a high rate of precision.

For example (exact stations will differ depending on airport and airspace type) a typical IFR sequence of stations.


Clearance delivery
Ground
Tower
Departures
Sector/area
...
Sector/area
Approach
Tower
Ground


The initial tuning will put delivery in live and ground in standby.  Then as you say, just press the toggle.  When the next frequency gets dialled in is not, as far as I know, governed in anyway and is probably not even in SOPs since it is just about using the equipment more efficiently.

Say delivery has given you a clearance.  They will probably follow up with something like "call ground 129.7" for push and start".  When you are ready for push and start, toggle to ground then put the tower into the standby frequency and so on.

Frequency changes are certainly made in cruise, when passing from one ACC sector to the next. These frequencies are published in the national AIP's and visible on en-route charts.  For a sector handoff, I'd probably dial the next frequency in when the current sector instructs me to contact xyz.

It's really not a big deal if you have to contact someone unexpected.  Modern boxes with push button tuning are almost as quick to tune manually as to toggle.