I'm fairly sure this would work on mesh bodies, but when I made system layer clothes (showing my SL age here lol!) I wore a skin made of the UV guides template on my avi. Then I could see exactly which guidelines and points matched at front and back, and spot any areas that distorted. Having the texture on the actual avi makes it much easier to see where you'd want to have a seam. You definitely don't want to have fancy details around seams until you get used to how things distort. I added a tiny blur of about 1-2pts along seam if necessary as that could often conceal the tiny seam glitches.
In Gimp I created outlines for clothing using the paths tool. You can create an outline path and it can be edited later, rather than a drawn stroke. This gets really handy when you want to change or build upon a previous design without starting from scratch. It also saves you having to draw the outline perfectly with a brush stroke, you can go back and adjust points and curves to reach where you want. The outline path is invisible until you apply a stroke to it, so I stroke the outline on its own layer. I'd drop a quick fill inside it (right click the path > path to selection, then fill) That makes it easy to see your outline when tested in SL. You need alpha in the test clothing outline so you can see your UV guide skin when you wear it. I'm not sure how that's all going to work with the mesh bodies, but it should be feasible!
I then used to wear my test layer over the UV guide and then if there are glitches, I can have SL zoomed in on my avi to see exactly what needs fixing. I use textures from my computer rather than uploading them at this stage - I think all the TPVs should let you use that.
Once it gets into patterns, you should find it easier to line those up testing as I outline above. It's not always possible to line them up perfectly, but if you look at clothing in RL the seam area distorts patterns there too. Also, there's nothing wrong with wearing a lot of patterned clothing from experienced designers and see how they seem to handle the seam area. Or use that 1-2pt blur and/or some shading. You can also build your design as if it was a bespoke tailored/embroidered design and deliberately make your pattern end before the seam. If in doubt, look at RL sewing/dressmaking sites and see all the different construction techniques that might be easier to fit into your designs.
I'm not sure if that all makes sense, but I'm happy to help further if you need it, I probably still have some "antique" SL clothes textures in the dark recesses of my PC to refer back to :)