That's a little unusual. The spelling E# helps my sightreading when the underlying harmony (perhaps) is a C# or A# chord, or F# major 7 - paricularly in big band piano parts (I detest those arrangers who spell a Db7 chord as Db-F-Ab-B instead of Db-F-Ab-Cb; it really slows down sightreading). But for ease of reading in an instrument part, I opt for the spelling that will be easiest for the performer.
Dean L. Surkin
Mack Brass MACK-EU1150S, BB1 (DE 101XTG9 mouthpiece in the drawer)
Bach 36B trombone; Bach 6.5AL mouthpiece (pBone on loan to granddaughter)
Steinway 1902 Model A, restored by AC Pianocraft in 1988; Kawai MP8, Yamaha KX-76
See my avatar: Jazz (the black cockapoo) and Delilah (the cavapoo) keep me company while practicing