I installed a digital voltmeter in my truck.

It is not switched and it is hot all of the time.
Should I just switch it, or wire it so it comes on when I start the vehicle?

If I wire it to come on when I start, which wire? 99 F250

 

Not sure about your 99 but my 06 has a circuit on the fuse box labeled as "Cust ACCESS" it's a 10 amp fuse and it is a capped white wire with a blue or teal stripe towards the middle of the dash just hanging there - kind of behind the ashtray.

 

Look in your fuse box at fuse positions #9 & #10. They're supposed to be unused. If the terminals are there, you can access the HOT IN RUN circuit easily by plugging into and accessing the supply terminal.
The '04 and newer are COMPLETELY different.

 

I expect that it'll take so little current so that discharging the battery won't be an issue.

 

Wouldn't it be easier just to go get a ScanGuage II. I have one and it shows real time voltage down to the tenths of a volt.

 

The ScanGauge is ugly.
The voltmeter was in the cluster that I installed.

 

Some seek fashion before form, others seek form before fashion, especially when there is an additional price tag involved. No right or wrong, all in the eye of the beholder and the pocket of the owner.

As the saying goes, different strokes for different strokers.

 

The ScanGauge might be a future option, as a tool with more features.
I installed a switch that will control both the voltmeter and a USB charging port, they both have LED's and I don't want them on 24/7.