Well, by now I'm going over my tree for something like the third time, still adding a bit around the edges - so it is easier to be systematic.
What I said about working by location is very much akin to working by source, isn't it? At least sources for me are by location, but I guess it depends on what kind of sources you have.
I don't think it's necessary for WikiTree to complete a profile all at once. I find that working up a branch for the first time it is usually easy to find births, which gives parents, then births of parents, which gives their parents and so on. Can be much harder to find out what happened to people at the end of their lives. Again, it depends on what kind of sources you have.
One thing with WikiTree is that I don't like entering people as Unknown. I tend to write the notes about them in the nearest relative until I have a last name at birth for them.
And I often make a list of children in the profile of the mother (to keep birth records in, for one thing) instead of entering each child immediately. That way I get an overview of what remains to be done. So here I keep the sources near to where I'm going to need them later.