It depends on the person and how good your prospective memory is. If you are the type of person that can "set" an internal alarm clock without fail and rather easily, then this may be all you need to MILD. Simply do your visualization a few times, drive your stake in (set your intention), and then think about whatever you want. If this does not bring satisfactory results, then you should probably do the whole procedure, mantra and all, or use other techniques during the day instead of the mantra.

Personally I never use mantra unless I am performing a WILD, though it is less like a mantra and more like focus. Good self-awareness, "state awareness", and intention is enough for me personally in the case of DILD. It will take some time to work out what works best for you, but be honest with yourself and don't be lazy. I wasted a few years being stubborn like that.