Standing tickets on Tohoku Shinkansen

Hi,

If I have a JP pass and unable to book reserved seating on Hayabusa/Hayate lines, do I need to book a Standing ticket using my JR pass? Or I could just hop on and show the train conductor my pass?

thanks

Yes, if you fail to make seat reservations for a Hayabusa, Hayate or Komachi train, then you have to get a standing ticket, instead.

I could just hop on and show the train conductor my pass?
I was not sure if it was illegal, but not knowing the exact rules, I once hopped on one, standing only (merely a few stations), showed my JR pass when checked, no question asked.

That's the wrong way of doing it. A lot of the time, the conductor will not care, but please do it the proper way and get standing tickets in advance.

I concur with Uji. Easy enough to obtain the standing ticket. I did that recently, my journey was less than 30 mins and did sit down for the duration as the train was practically empty.

See the railmap and learn about basic info(timetables and more) first.
http://www.jreast.co.jp/e/routemaps/tohokushinkansen.html
https://goo.gl/SDK6by
https://goo.gl/JKWW6i

Btw Tokyo and Sendai, standing ticket is necessary bc reserved seat cars only.
Btw Sendai and Morioka, only trains that doesn't stop at the each station are necessary.
Btw Morioka and Shin-Aomori is not.
But for many passengers getting off at Sendai usual
as down route(for North) trains you may book reserved seat easier,
and also operated enough "Yamabiko" that have non-reserved seat cars until Morioka,
just this will be more than stops, slower than Hayabusa and Hayate.

In Tohoku Shinkansen, standing ticket is two different types .
a) It is released only when already full and can not sit any vacant seats,
for btw Tokyo and Sendai or Morioka(only for non stop type trains).
b) It is always on sale and can sit any vacant seats only in ordinary cars,
for btw Sendai and Morioka, Morioka and Shin-Aomori,
but must move to another vacant seat when a passenger who reserved the seat where you sat.

It is necessary to buy a) or b) beforehand in the regulations,
but PASS includes the effect of either ticket, no penalty if you couldn't buy it.

And, "Komachi" is the train name of the Akita Shinkansen(E6, Red colored one),
http://www.jreast.co.jp/e/routemaps/akitashinkansen.html?src=gnavi
in Tohoku Shinkansen section, all trains are operated with "Hayabusa"
btw Tokyo and Morioka. (only two trains are btw Sendai and Morioka)
If btw Tokyo and Morioka, you can also book the reserved seat if not full.

There are plenty of trains that have non-reserved seating where simply getting on with a JPR is quite allowable. If you are using a service that is all reserved seating, then you need a ticket to board (reserved seat or (as you inquire) a standaing seat if available when all reserved seats are sold.
You could (or may need to) change your travel time.

will there ever be a case where even standing tickets are sold out?

Yes.