I use 12 o'clock for safe zone. As far as turning the cam with the chain off, that is ok. You just shouldn't turn the crank with the chains off. But you can rotate those cams round and round as many times as you like and it wont throw anything off. I never once used TDC for anything because for some reason I could not get it right. I used keyway at 12 for everything. And when everything is set, you will have tension and then slack in the chains. That is normal. It will have that slack until the tensioners fill up with oil. And make sure that the timing chain is marked. If it isn't marked already, then mark it yourself. Mark one end and then the exact opposite end. You can do that if you lay the chain out on a table and mark the exact opposite ends. And then count the links to make sure they are exact opposites.
The timing marks on the cam sprockets might not always end up picture perfect. But just make sure that you use proper torque on the bolt and do not go higher than that. As long as the chains are correctly on the timing marks of the crank sprocket and the cam sprockets and there is tension (with some slack as you turn it) then you should be fine. You don't want it to be a lot of slop or slack. But you'll see it slack a bit when you turn the crank sprocket. Sorry if this is confusing but its one of those things where I can do it but it is more difficult to explain. I'll be doing a cam swap myself this weekend.