Tsuki is actually limited to Dan grades only in competitions. There are no official rule says u cannot do it during practice. But a unspoken rule in kendo is that do not use tsuki against kyu players without telling them first. at least that's what i've been told in my dojo. That is to avoid using tsuki when u are not very experienced in executing the cut. tsuki can be a very very dangerous cut if not doing it right and really.. no one will think nice of u after u do that to them during practices without regardings cuz it'd be painful and quite scary. think if someone does the same thing to u wat'd u think?
just my 3 cents