With hairlines there is no one size fits all. Bone structure and degree of loss dictates what is wise and fitting to design.
Bone structure and loss can result in the recipient area being larger than the donor area and this being the case it is not wise to design a low flat hair line because it is important to restore a facial balance, not just from the front elevation but sides and top. By restoring a low frontal hairline you may create an unnatural facial proportion.
If the hair line does not match the bone structure in relation to the patterning of hair loss this can result in a front loaded design, many grafts being used to restore an area and the creation of an unnatural pattern of hair growth, high frontal density and a design not fitting the patterning of hair loss behind the frontal, compounded with a larger area to treat if and when the loss progresses.
Ethnic characteristics can also dictate changes in a natural hair line so must also be considered with design.
Above all it is important not to have a designer hairline but rather something that will suit your own personal features.
So, whether 5,6,7 cm hairlines are good is only a part of the overall picture after considering many factors.