the length of your program/number of frames is the same when using drop vs non-drop. Drop frame was created to allow television programmers to work with hours/minutes/seconds, and forget sub-seconds timing. Frames are dropped at regular intervals so timing makes sense to the broadcaster. Drop frame time code drops two frames every minute except every tenth minute.
Non-drop is almost always used unless on a broadcast master. This is for video.
For audio, it's nearly always a straight 30 frames, but that's changing.
However, NTSC standard is 29.97. Sometimes people (including me) will say 30 frames per simply because we're too lazy to spit 29.97 out of our mouth.
Anyway, the formats you'll use to drop T/C on a review copy of a video should be non-drop. If you are sending a master copy to a broadcast house, you should use drop frame.
if you are working on a 24p timeline, you should use the 23.976 IVTC setting.
Any other T/C setting you might use as a visual aide is for in-house reference, or for a musicians reference.