It is a matter of personal choice how many direct lines and their collaterals are investigated. Computer facilities have enabled the scope to be widened compared with written records, but the time available is still a limiting factor. The problem is that the number of lines doubles every time you go back a generation. With limited resources some people concentrate all their efforts on the line bearing their surname. Others follow the four basic lines of their grandparents. Unfortunately, the contribution of these four lines to one's genetic inheritance is halved each time we go back a generation. Prior to the 8th ancestral generation, the genes transmitted from those four lines to the present generation are less than 1% of the total; (i.e. the number of ancestral lines 9 generations ago is 29 = 512 and 4/512 is 0.78%). However, there are some genes and determiners of inheritance which are not diluted as the generations pass.
In the male-male-male line the Y chromosome and its genes are passed on to males only, unchanged. Similarly, in the female-female-female line the genetic determinants for the mitochondria (the self-replicating organelles, in the cytoplasm of cells, responsible for energy production) are only passed from female to female and female to male. Since mitochondria can not be transmitted from the male through the sperm, the mitochondrial 'genes' can not be passed from male to male or male to female, but can only come from the female line. A novel approach to genealogy would be to concentrate entirely on these two lines. In this way you would know that you have received the Y chromosome (if male) or the mitochondria (if male or female) from distant ancestors unchanged.
Actively collecting collateral records beyond third cousins (or equivalent) is wasted effort; although if such records have already been obtained by others they should be retained as a useful data-base to check on wider family links and inbreeding. The number of generations one goes back in the direct line is limited only by the records available, which are very sparse before the 17th century. Really keen genealogists will research not only their own family trees but also their spouses'; the combined result being their children's family history.