If you hit 30º exactly you are making a perfect thread. if you happen to be a bit over, well, it's not so good, that surface (on the right) will be stepped. If you aim at 29.5º and miss by a tiny bit, it won't matter. I believe this is the true argument for 29 1/2º. The EXACT angle doesn't matter, just so it isn't over 30º.
There are as many different ways of doing things as there are machinist, but I cut my teeth on South Bend's book on How to run a lathe and here is what that says on this subject. There is more than one way to skin a cat... but this has worked well for me over the years on the lathes that I have run.
Subtract one from that camp; From the time that I took machine shop in HS from a teacher that apprenticed at Mare Island Navy yard, and taught in the apprentice school, and was subsequently apprenticed in a large local shop, I never knew of any fellow machinist since that did the 29deg. thing. The only place that I hear about 29deg is on this forum; I am now going on 75 years ago, and can say that 30 degrees works just fine, with no incidence of steps on the backside of the thread. So far as feeding straight in, except for very fine pitches, it is not a good plan, as the chips coming from both sides of the cut interfere with each other at the center, and can cause finish problems. going straight in can cause problems with remembering dial readings.
As an aside, the Europeans have the custom of feeding straight in with the cross feed, but feeding in with the compound from right to left with each succeeding cut, this told me by a German guy in our shop, Karl Groneck.
Since I removed my top slide and replaced it with a solid plinth I have to go straight it.
I slightly round the point of the tool do the thread bottom isnt a sharp "V"
For my use this has been a simple process for all the threads I've cut to date BUT 2mm between crests has been the largest thread I