You need only split a task into as small a chunk as you need to, to make sure that you have a clear idea of what to do.
As long as the next action is clear, specific and doable, you should have no problem.
For example, "buy a car" is too vague as a next action, but, look at ads in newspaper would be OK for me, even though it skips steps like 1) pick up the newspaper off the table, 2) find the index, 3) look up what page the ads start on, etc.
Some guidelines I use:
1- Will I be able to do the NA on the list without any decision-making first?
2- Is the NA an action (with a verb)?
3- If a NA will take less than two minutes, do it rather than enter into a next action list.
4- Use specific language ("send email to Joe" rather than "contact Joe")
-Ken