It has been confirmed by Ryan that it is OK to use letter and word dashes eg.

_ _ _ _ or _____/______


However I ask many of our experienced players to try to draw with out using these, as it makes guessing too easy for advanced levels. Give yourselves a challenge, it can be rather fun to see if you can convey the word without using any hints. As you play more and become more experienced, most of the time it is easy to guess words without any hints anyway.