When may a pilot cancel the IFR flight plan prior to completing the flight?

When departing from an airport located outside controlled airspace during IMC, you must file an IFR flight plan and receive a clearance before

What altitude may a pilot select upon receiving a VFR on Top clearance?

(Refer to figures 41, 42, and 42A.) Approaching DFW from Abilene, which frequency should you expect to use for Regional Approach Control, control tower, and ground control respectively at DFW?