NP

If I do not do this, and I do not load the Dawn Guard esm, will I be able to make mods that will work for people that do not have Dawn Guard yet?
Or as long as I do not use any resources from Dawn Guard I do not need to worry about it right? Just like with Shivering isles and Oblivion?
All should work normally if DG isn't loaded, yeah. If you want to fork code around its presence/absence, you can use *http://www.creationkit.com/GetFormFromFile_-_Game#Examples. No master dependency is necessary.
*Turns out the example will work as there is an 0x00000800 form in Dawnguard, DLC1AurielsBow "Auriel's Bow" [WEAP:02000800]