Richard,
I too have built quite a few "vintage" wheelsets without a tensiometer and after many thousands of miles I have not yet broken a single spoke while riding nor damaged a rim while building. As Mike Kone remarked, the trick is really to get the tension as even as possible all around.
In the late 1960s I had a wheelset built by a fine wheelbuilder. I watched as he simply "plucked" the spokes just as you might pluck a guitar string, The sounds made by each of the spokes will give you a pretty good sense of the relative tension of each spoke.
This method still works well for me. I'll later check the tension with a meter and sure 'nuff, with a decent musical ear you can easily get the tension within 10% - which ain't bad.
I use the Wheelsmith Spoke Prep a lot now (probably just because I'm a sucker for their high-tech hype), but the linseed oil is also fine... and MUCH cheaper. It lubricates and then gradually will get sticky as it slowly dries - a good thing, since this helps lock in the spokes. But, you really should use something on the spokes; it makes everything easier.
One last personal suggestion I would offer is to lube the eyelets of the rim. I use a simple (and probably frowned upon) procedure: dip a Q-tip cotton swab in a gob of vaseline and spin it on the inside (tire side) of each eyelet. This helps eliminate the occasional loud and unnerving squeals from the friction of a spoke nipple being tightened against the eyelet - and it's especially useful if you're using a vintage or even a 'used' rim [my bad] which may have a minute build-up of corrosion or just dust and grime inside the rim and eyelets.
DT and Wheelsmith spokes are excellent products, technically far superior to vintage spokes. But, I have built many wheels using both old Robergel and Stella spokes - all those spokes double butted, and both brands in now uncommonly thin gauges... which is now generally advised against for rear wheels. I'd purchased a fair stash of those spokes several years back from Mike Kone when he was Mr. Bicycle Classics [Many Thanks Mike!]. Even those delicate looking spokes have held up just perfectly, in spite of having traversed far too many pot holes, railroad crossings, bumpy dirt and gravel roads. I think just as long as the wheel is carefully built to begin with, the quality or the thickness of the spokes matters less.
It's truly very satisfying when you have ridden thousands of miles on a self-built wheelset and suddenly realize that you have never had to tighten a single spoke or re-true a wheel in all that time. ~ I'll shut up now before I rant on prosaically about the Zen and the holistic health benefits of wheelbuilding.
Just take everything slowly, very slowly. It really beats having to re-start when you notice you've made just one small goof somehere back down the in the process.
Bob Hanson, Albuquerque, New Mexico, USA