Why you make your boards at 10 if 5 worked? Was 5 more effort or some chance of failure in resist or etch? (I assume the manufacturers charge more for sub 7 or so mil traces for actual reasons.)
The way I line up a double sided board is by drilling two holes in the papers. Transfer one side and drill the pcb through the 2 holes in that paper. Then put the second side on and line up the holes. Stick that side. Then remove both papers and etch. I don't know what kind of accuracy you need, but this isn't hard or iffy to complete. It is just time-consuming. You save a lot of time elsewhere, though.
If this is a DIY board, only, I might enlarge my vias. But if it might go into production, I will just work with 15 mil vias, 8/8 default traces. The drill holes might break the annular ring on one side, but I will get everything working just fine. So I actually make DIY boards almost exactly the same as production boards, no problems (although I may increase the clearance around pads, due to lack of a soldermask layer; and I can't put vias under SMD IC's). Using 8/8 traces is not "showing off." It's the size that is easiest to route for me, and also once you find the smallest size of traces that does not cause additional issues, you are making your board MORE reliable by reducing the area of the board. The larger your boards, all else equal, the more chance of a failure. With the right process (I assure it is easy), 8/8 is just as reliable as 20 mil traces. So this allows more complex and/or larger boards without adding more work, prep, scrubbing, or chance of a failure due to lack of attention over that larger area.
You can have issues with many toner transfer papers due to distortion. There's no one forcing you to use these inferior papers, though. Magazine paper was the worst, I tried. Enough distortion to cause problems on larger boards. But there are other reasons I would not use magazine paper, anyway.
In short: 90% of people who use UV, it's because they learned toner transfer from internet hacks, and they (quite reasonably) gave up before sorting through the different options, from "make a crappy PCB for free with shit that is in your house right now" to actual pro-level toner transfer. For most people, toner transfer is the better way to go so long as you don't try to cheap out. UV might be cheaper to get into, initially, esp if you don't have a suitable laser printer. But toner transfer is easier and faster in the long run, and in that long run, both methods are dirt cheap. The labor is what matters.
If you are the kind of guy that will spend $100 dollars on various mystical photo papers trying to find a lifetime supply of pcb paper for cheap, well then you can waste years learning subpar toner transfer, trying to convince yourself your latest underground bargain score paper, learned from some dude on the internet, will eventually work perfectly with enough practice. I'm guilty. I got through it, though. I also come to believe that most of us don't know what "perfectly" means when we say it. When we sometimes get something that looks very much like pictures what other people get (from 5 feet away), we assume we "got it." Making a PCB is about reproducing fine details reliably. You have to pay attention to details and consistency to know if your way is "the best."
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