should i put my Nintendo switch on sleep to save battery ?

It depends on how long you're going to be away from your Switch & how much time investment is required to make significant progress.

If I'm going to be away from 5 to 15 minutes, I'll definitely throw it into sleep mode so I can jump back in quickly.

If I'm going to be away for 30 minutes to 2 hours & the I've already invested about 30+ minutes without a viable in-game save point in sight...  I'm going to throw it into sleep mode in hopes to pick up where I left off when I can.

If I'm going to be away for 2+ hours & can get to a save point quickly.  I'll just save & power down the Switch as to save the battery until I'm ready to pick things up.

The TL;DR is that putting your Switch to sleep is better than just leaving it idle when you have to step away for a short time.  If you're going away for a considerable amount of time, you're better powering down instead of going to sleep.  HOWEVER, it's ultimately your decision on how to expend your Switch's battery when you're on the go.

If you're Switch is docked, then send it to bed (& put it to sleep) when you're done playing for the convenience (& to allow it to download updates).  Only turning it off if you're taking it out of the dock & not playing it in the near or immediate future.

What's the alternative?  Leaving it on the entire time?

If so, then yes.  Sleep Mode will save battery.

If the alternative is to fully shut it down, then it won't save battery by putting it into sleep mode.

But I very rarely power down my Switch.  I do put it into Sleep Mode when I'm not playing it.