That is a good question. If the sound of a phonogram is not given on the phonogram card, the phonogram is underlined twice. Look on WRTR pages 44-45 under the heading Spalding Marking System. The i in machine (WRTR pages 333 and 382) is underlined twice because it is an uncommon sound in that word. The x in xylophone is underlined twice because it has an uncommon sound in that word (WRTR page 397). It is also very helpful to always read over the Instructional Tips in the third column of the Extended Ayres Word List prior to spelling dictation.
In other words, whenever you get to a word which has an unusual sound (in the Extended Ayers list) that is when you introduce it.
Lorene Peetz
"But let all those rejoice who put their trust
in You; let them ever shout for JOY, because
You defend them; let those also who love
Your name be joyful in You." Psalm 5:11