On my Roland MC50 I can set the metronome to sound when recording, playing, or stopped.
In Stopped mode, the metronome sounds all the time and I can sing and turn the dial until I think the tempo is right.
 
In Sonar I can only set the metronome to sound in record or play mode. There is no "Stopped" setting.
 
In Play mode it only uses the start tempo. If I change tempo after I click Play, the tempo does not change.
(note: I have not recorded anything yet. All tracks are empty.)
 
I found a tap-tempo button in Project > Insert Tempo Change, but you cannot hear the metronome
there and I cannot sing against anything to see if it's right. All it shows is the tempo number.
 
I am using my hardware metronome now, but does Sonar have a better way?

Try this:
 
- Disable Stop at Project End in Track View Options if it isn't already so you can run the transport in an empty project.
- Start playback with no click sounding, count off a couple-few measures in your head, and stop the transport on the next downbeat.
- Shift+M to open Set Measure/Beat At Now, and enter that measure and beat.
- SONAR will set the tempo to make the specified beat fall on the absolute Now time where you stopped the transport.

I mapped the Project -> Insert Tempo Change to my Alt-T key. Then when I need to set a tempo for a new song, I press Alt-T and click on the Tap button while singing the song. When I'm satisfied with the tempo, I click the OK button on the tempo popup and the tempo is set.
If I want to check the tempo with the click, I click on the Metronome During Playback and play a few bars before recording. No need to enter any numbers. I just go with what feels right for the song.
I use the same process if I want to change the tempo during the song. For anything more complex, I go into the tempo map and make my adjustments there.

Works!
It's counter-intuitive at first, but faster and more natural because I'm just clapping and singing the song in my head.
 
Truth be told, the MC-50 metronome actually puts me off, makes it hard to let the natural rhythm of the song come out.
 
There's one gotcha: my song starts with an up-beat. So it's "da da da BAMM..." - the BAMM is actually the first note in the first bar. I needed a coffee break before I realized why it wasn't working. Hit the spacebar on the BAMM, not the first da.
 
Great idea, thanks very much!

@DPete - A good one. But I have never been able to do tap tempo. Clapping is fine, but put a switch, mouse button or spacebar between me and the beat, and I can't do it.

Cool. Glad you like it. It also works nicely to match a tempo that someone else counts out or plays on their instrument.

I put in a feature request some time ago about a variable speed metronome that would allow you to dial-in a tempo. As I recall, nobody gave it a sniff, but I'd still like to see it.

Amp fixer, this would be great for jamming with friends.

I have an old friend I hang with who has a hardware drum synth and when he gets bored he starts twisting the tempo nob. Changes the vibe in a hurry.

I would also like to see a friendlier touch to change tempo. I often practice to midi tracks and slowly speed the tempo as I learn the piece.

I don't like to always practice at the same tempo because then I have trouble adjusting if the tempo varies live. That means a quick screen touch to alter the tempo slightly is really helpful.

I think it would also help when you try and add a click to something that didn't originally have one. I've tried a number of high tech ways to determine tempo from a live track but a manual metronome with a dial works best for me.

Since I have EZ drummer in most of my projects, I use the browse function with "Follow ..." unselected.  The auditions loop forever and it's easy to change the tempo while it continues.  Once I like the pace, I stop it and enter that tempo for the project, reselect "Follow ..." and I'm good to go.
 
I've ALWAYS preferred drums to metronomes, especially if the drums are in the basic style I have in my head.  But even Kick-snare-kick-snare is better than a metronome for me.