allegiant airlines carry on bag dimensions is 9" high 14" wide 22" long does it have to be exact?

I wouldn't attempt it with a gyppo airline, and Allegiant is as gyppo as airlines get. The extra 2" in height may prevent it from fitting in the overhead compartment on some planes.

The dimension guidelines are so that the bag fits safely in the overhead or under the seat. It is a safety concern, to make sure that in the event of an emergency, your bag is not flying around. If it looks like your bag is not within the dimensions required, they will have you try and fit it into one of those bins that they have that match the proper size compartments, and if it will not fit in even when squishing it, they will make you check it. A little difference won't hurt, but a couple inches in the wrong direction could put it over.

why risk it in the over head compartment check it in with the regular bags .