It would be interesting if people mentioned where they live (or grew up).
Anyhow, it’d always unvoiced th for me (in the Northeast US).
My guess would have been that it’s a matter of sociolinguistic register or prestige rather than geography (and namely, that the voiced th was considered the high-prestige variant). Hence, I was surprised to see that some people, in formal contexts, take pains to use a voiceless th where they would ordinarily use a voiced th.
Always the voiced. Chicago area, Canadian influence to speech from my mom.
I don’t understand how you could have a voiceless th in with or myth. What would that sound like, “wi” and “mi,” or the babyish “wit” that little kids are corrected for? With and myth both have a voiced th sound at the end. All the derivatives in questions 2 and 3 do as well.
The voiced doesn’t refer to whether you can hear it. It’s referring to whether you are using your vocal cords. Are your vocal cords still moving when you get to the end? Mine aren’t. I say “wi” using my vocal cords, and then “th” using just air through my teeth.
It’s the difference between “bus” and “buzz.”
Oh, and unvoiced for all, so far as I can tell.
Voiceless means that the th is pronounced like the th in thin or brothel (or, of course, the usual pronunciation of myth). It does not mean that the sound is silent, although the term “voiceless” might suggest that (rather, voicing refers to vibrating vocal cords–place your fingers on you throat and compare the minimal pairs thy, a voiced th, and thigh, a voiceless th).
ETA: Ah, I see jsgoddess has already addressed this topic.
I feel so low-brow.
Well, I only guessed that because pronouncing withstand using a voiced th seems to me almost a parody of excruciatingly correct enunciation. Try saying it yourself: “I cannot withstand your effrontery any longer! You shall leave at once!”
South Australia, FWIW
Sure, I use it so often, but it’s still difficult for me to properly ascertain how I say it; thinking about it consciously isn’t necessarily leading me to accurate answers. I think my answer would be the same as cjepson’s, though.
Always unvoiced.
I wonder if people who have posted actually say these words the way they think they do.
Asking someone how they pronounce something is a good way to hear how they don’t pronounce it. Gotta trick them into saying it 
So, no idea how I say it. Maybe I’ll hear a recording of myself some day and find out.
With, withstand, withdraw: voiceless.
Within, without: voiced.
NYC, if we’re keeping those stats.
Always voiced in all cases, except for forthwith. I’m from northeast Ohio.
Usually voiced. Unvoiced in forthwith, and possibly in withstand. I think it is a regional thing rather than a prestige thing. The prestige variant is more the wif/wiv, wit/wid vs wiƟ/wið. Just a guess, though.
If the following sound is voiced, it’s voiced…I think. “With gusto” isn’t voiced, so I guess that breaks my rule. But “With the” is certainly always voiced while “with care” never is.