Question:  Is it OK to put a sticker or decal on my rear car window before having the windows tinted.

It depends on which side you put the sticker on. Because window tinting is done on the inside of the glass, it would be a poor choice to put a sticker on the inside of the glass before the tinting is done. The person or company applying the window tinting will let you know how many days the tinting should be on the window before applying the sticker to the window.