I use the paddle shifter mostly for downshifts if it 'D'. However if I was to try to lock it into a gear I would put it into DS so it doesn't shift back after about 5 seconds of straight line travel.

 

No need to put it in DS for this (for me at least). 95% of the time it holds long enough to do the trick in D, even very long descents, it seems to know what I am doing and holds 5th for an extended amount of time. On flats sometimes (the 5%) it does switch back to D quickly, but one more tap of the + does the trick, or I just leave it in D as like I mentioned before I don't think this really does much.

 

If I remember correctly it stays in for something like 5 seconds unless it senses otherwise (such as going around a corner, etc.). So it may be that going downhill will hold it in as well. But if I wanted to keep it in I would typically want to lock it in. Seems like it would be just as easy to place it in DS and place it in 5th for this purpose.

On the other hand I actually do something similar. When I enter a round about I typically will in 'D' put it in 2nd or 3rd to keep it from doing a strange downshift on its own. It will stay until after exiting the round about.