Mastering & Digital Downloads Only

Well that is an interesting one because I have been referring to burning CD's and the silence is actually inserted in that process.
 
If you are concerned that digital downloads are going to be played end to end eg butted right up against each other then the obvious place for the silence is at the end of each track. The only issue is that if you decide to burn a CD with these tracks that have silence at the end of them you will need to make sure no silence is inserted when burning the CD.  So the tracks do play end to end but the silence at the end of the tracks will create the silence between.
 
 
 
 

I think it's common practice to insert a little silence at the end for downloads. I've just had a look at a sampling of songs I'd downloaded from Amazon. I don't have many, as I usually buy CDs and rip them. But all the ones I looked at had about 3 seconds of silence at the end.

Digital downloads are usually ripped from the CD.  The silence at the ending of the track from the pause marker to the next track is usually added up at the end of the mp3 file.  
There will usually be a pause between songs, especially with streaming, due to the time it takes to load and buffer the next track, one reason I hate mp3 downloads. This will add even more time if you wish to listen to tracks back to back.  If you are able to master directly to mp3, it would be best to judge how long you want the next track to come in and subtract a couple/three seconds from the silence.  Just a thought.