Can I replace the [ð] sound with a flap [d]?

[fruitninja]

To me, the [ð] and us flap [d] ( as in better), sound fairly similar. If the [ð] comes at the beginning of a sentence, e.g. that's right; I have no difficulty saying it. But, if it's somewhere in the middle; e.g. what's wrong with it? or See you then; I find it a little hard to say it smoothly( doesn't flow). So, in those sentences, can I use a flap(or soft) [d] instead of a [ð]?

 

[GoesStation]

Although that substitution is normal in some dialects and will usually be understood, you will have the best chance of being understood if you can master the ð sound.