While using double quotes works, there are certain circumstances where this won't give what you want. For example,
echo '123$tbcd' | sed "s/$t$t//"
(yes, this is somewhat contrived!). You can avoid this by either escaping certain characters:
echo '123$tbcd' | sed "s/\$t$t//"
but it is easy to miss this. The safest way, in my opinion, is to surround the variables with double quotes (so that spaces don't brake the sed command) and surround the rest of the string with single quotes (to avoid the necessity of escaping certain characters):
echo '123$tbcd' | sed 's/$t'"$t"'//'.