In my experience, the alignment marks on the rear swingarm are not particularly accurate. I found that after performing a careful front to back alignment on my 78 750K that the marks were off by couple of increments. Using the marks and eyeballing might get you close but if you want to optimize handling then its probably worth doing a proper alignment.
How did I do the front to back alignment? Straight edges would work fine but sourcing the the materials ( I have read some recommend long flourescent light tubes which are perfectly straight and perfectly fragile!) and setting up is probably easier with good old string lines and the results are as good if not better. Dental floss works well! Basically you bring the string tightly around the front and rear tires (about 1/3 of the way up) forming a complete band. Bike is on a lift or on its center stand and the wheels must be stopped from rotating. Cut a pencil or something similar (light weight) to the exact width of the rear tire and install it between the taught strings up close to the front wheel. Tension will hold it. Get the front wheel pointing as close to straight ahead as you can and fine tune by getting the distance between its rear sidewall and the string the same on both sides. Now look at the gaps between the string and the rear wheel's leading side wall. Unless you are right on, one side will be touching and the other will be gapped. Adjust the rear wheel until there is no gap on both sides (your carefully cut pencil is holding the strings apart at the exact width or the rear tire so when the front and back is aligned there will be no gap) Now if your rear alignment was off a good bit initially you might have to tweek the front wheel centering a time or two as you get things closer and closer. The trick is to get your chain tension and alignment both correct at the same time after the axle bolt is torqued. Lots of trial and error. You could also simply check the accuracy of the swingarm's marks and make a note that for instance the left side should be one mark more than the right. This won't be as accurate as there is some built in slop in the chrome chain adjusters and axle but it should get you close. This method was a nice final step in getting my bikes handling as crisp as possible. It is certainly a waste of time if things like your swing arm bushings, wheel bearings, wheel runout, tire seating etc etc is not sorted out first. It is also basing rear wheel alignment on maximizing handling performance rather than chain wear. Some align to get the sprockets lined up but IMHO thats not what riding is about!
