I wouldn't get into the habit of tensioning and de-tensioning strings too much. It can interfere with the harmonic properties of the string. Also, you will get flat spots where the frets have been and these will hardly ever line up.
Some people take old dull strings off and boil them for a while to make them sound brighter. When I've done this I get almost a chorus effect out of the strings when they are back up to tension, and they don't last that long either.
What makes strings go dull is sweat and bits of skin etc getting into the grooves of the string winding. When you take the strings off the dirt in the grooves gets compressed, when you tension again the grooves get slightly larger so the compressed dirt loosens and falls out. Boiling them gives a deeper clean but the changes in temperature also deteriorate the sound properties of the string.
You could try cleaning the strings, under tension with isopropyl alchohol (don't use whiskey!). You could also try wiping the strings with a lint free cloth after every time you play the bass to prolong their life. Some people make strings go dull quicker than others, it depends on the acidity of your sweat.
When i was touring a set of steel elites would last me two nights before they turned into rubber bands! I use the D'addario EXP coated strings these days, they are not cheap but they last for ages, the only ones I've tried that last longer are the DR Bootzillas but they are super expensive and have quite a harsh sound.
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