If the Tru-Oil feels dry, usually it is.
I usually add a few coats of Tru-Oil and let then dry(usually a coat per day), spray satin poly where I want to write, let that dry, write on it and Tru-Oil over that to seal it all in. Five to 6 coats all together. I've used Tru-Oil for many years, on many bows and never worried about it affecting cast, draw weight or affecting causing set.