Take it of they offer it so you have it if you need it, but don't be so quick to text them or give them yours. On a slow day you and every other customer that she has the number of will get the generic, "Hey Baby, I miss you, when you come see me"
Never give your real cell number, always give your google voice or whatever other burner app number you are using. More than likely, she is giving you the same. Some app number and not her real everyday cell number. For every girl that might be genuinely interested in something more, there are another 10 that will just text you when she is slow.