I know this is a little late, but I just saw this discussion and would like to add my 2 cents. If you're building an unfretted guitar you'll be fine, but I strongly believe a fretted instrument should be built with a 2 piece laminated neck if you;re not going to use a truss rod. To achieve this simply saw your neck 1x2 right down the middle, give each piece a quarter turn, then glue back together. This has several advantages. Aside from making the neck stronger because of opposing grain it also makes your neck straight (it's damn near impossible to find a perfectly straight 1x2). It will also make your neck a bit thinner (and more comfortable to play) because you've removed the thickness of the saw cut. After gluing the two pieces together run it through a planer to make it flat then glue on your fret board and you'll have a much stronger neck than one piece of 1x2 that will also resist warping over time much better. I've been building my fretted necks like this for years now and can set the action quite low if I wish. So far none of my necks have warped. BTW I will only use maple for a neck.
It also depends which wood you use for the neck. Straight, unblemished hardwood, like oak, for example, you won't need a truss rod for reasonable scale lengths (21-25.5").
Poplar or pine with high string tension, you might need one, as they are softer woods.
Also, make sure the wood is actually dry, with no twist. Some slight bending along the length is acceptable, if you place the bend toward the back of the neck, so that string tension pulls it out.
Well, it all depends on the balance between your string tension and neck stiffness and neck angle. With sufficient strength in the neck and perhaps a little back angle, you can certainly get by without one. Also, if you're playing slide (much higher action) rather than going after a really low action, you can probably skip it for sure. Tell us a bit more about your build and playing style and I suspect the many wise guys :-) on here can give you some great input.
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I know this is a little late, but I just saw this discussion and would like to add my 2 cents. If you're building an unfretted guitar you'll be fine, but I strongly believe a fretted instrument should be built with a 2 piece laminated neck if you;re not going to use a truss rod. To achieve this simply saw your neck 1x2 right down the middle, give each piece a quarter turn, then glue back together. This has several advantages. Aside from making the neck stronger because of opposing grain it also makes your neck straight (it's damn near impossible to find a perfectly straight 1x2). It will also make your neck a bit thinner (and more comfortable to play) because you've removed the thickness of the saw cut. After gluing the two pieces together run it through a planer to make it flat then glue on your fret board and you'll have a much stronger neck than one piece of 1x2 that will also resist warping over time much better. I've been building my fretted necks like this for years now and can set the action quite low if I wish. So far none of my necks have warped. BTW I will only use maple for a neck.
Poplar or pine with high string tension, you might need one, as they are softer woods.
Also, make sure the wood is actually dry, with no twist. Some slight bending along the length is acceptable, if you place the bend toward the back of the neck, so that string tension pulls it out.