It is better to state in the petition letter about your eligibility of reqested lenghth of extension. However, as long as the I-129 is correctly filed and all required documents are there. You should be OK. In case a denial, such a denial is not difficult to overturn by filing MTR.
The following part of memo may also help you, just peace of mind
Question 2. How early can a request for an H-1B extension beyond the 6th year be filed?
Answer: The April 24, 2003 guidance memorandum is modified in the following manner: a petitioner must establish that the above criteria (see Question 1 outlining requirements under Section 106(a)) were or will be met either on or before the requested start date on the H-1B extension application. Thus, an alien is eligible for an extension of H-1B status beyond the 6th year as long as either the qualifying labor certification application or I-140 petition has or will have been pending for at least 365 days prior to the alien’s requested start date, regardless of whether the H-1B extension application was filed prior to the passage of such period If the alien would no longer be in H-1B status at the time that 365 days from the filing of the labor certification application or immigrant petition has run, thus leaving a gap in valid status, then the extension of stay request cannot be granted.