According to the rules of Hepburn (http://www.halcat.com/roomazi/doc/hep3.html), しいたけ is correctly romanized as shiitake. What is the correct romanization of イー as in シート? Should it be shiito or shīto or shi-to?
However, a vowel lengthened with the 長音 「ー」 (usually in loanwords) is romanized differently:
a macron over the vowel before it, e.g. シート → shīto
(See Kenkyūsha's New Japanese-English Dictionary for modified Hepburn)
a circumflex over the vowel before it, e.g. シート → sîto
Wāpuro rōmaji (or wa-puro ro-maji)
a hyphen after the vowel, just as is typed, e.g. シート → si-to
N.B. In other systems the hyphen may be inserted for readability.