So I picked up this old 60s Raleigh frame and built it up a few months ago. I had to file the front drop outs for the wheel to fit up front. No problem. From day one of riding though the bicycle has been veering slightly to the left when uncontrolled. Is this common for conversions? Is this an issue of any importance? It's only significant when I have to do something briefly that requires no hands. I have to lean awkwardly to counterbalance. So, if I were to file down one of the drop outs a little more would that be a good solution?

That happens to me sometimes. Just make sure your wheel is on straight. If you can't, then yeah, finish filing the dropouts.

i say true ur wheel just in case

is the rear wheel centered? if it is off to the side it will drift because of that too.

no, it's not common for conversions (or any bicycle)
and yes, it's significant, because every time you look behind to check your six, or traffic, your going to drift to your left. (not good)

I can ride my conversion no hands for miles, and even while looking behind me and it goes straight as a pool cue.

something obviously isn't lined up right, or your headset has a "set" in it.

check the alignment (measure), check the headset, check the axles, check the wheel true (or dish).