You can leave the 5th string uncapoed when playing in other keys. However, one caveat: it may create a dissonate sound when you hit the 5th string. Sometimes you can get away with it if it's a fairly fast song. One idea for an uncapoed 5th string: stay away from it.

Do you have rr spikes on  your banjo?  If not, you might think about getting them at 7, 9, 10.  I've got spikes a 7, 8, 9, 10 and 12 (don't use this last one).

Having to continually retune the 5th string at jams isn't usually possible as by the time you get retuned, the song is about over!

The 5th string is typically tuned to either the tonic (e.g., G major using gDGBD), the fifth (C major using gCGCD), or less commonly the third (D major using f#DF#AD). In these cases, the 5th string is tuned to one of the notes in the tonic chord, not just one of the 7 notes in the key. That's not to say there aren't exceptions... 

I've been playing in jams for a very long time. I chose to run a spike at A and have never needed anything else. I usually capo to play A & D tunes.

All other "singers" keys (Ab, B, Bb, E, Eb), I simply avoid the 5th string.

When I'm playing primarily backup, I stay tuned to gDGBD regardless of key. Again, I just avoid the 5th unless it is needed or fretted.

Thanks.
I can capo the fifth "relatively" easily. But avoiding the fifth string is probably easier.
That makes some sense about tuning the fifth to one of the triad notes in the tonic chord.

Just my experience, in case it adds anything to this thread. My banjo is tuned standard gDGBD all the time, and I’ve never played in any other tuning. Coming from guitar, bar chords and partial chords are no problem, so I’ve never used a capo. I’ll play in any key required, using bars and partials and inversions as needed. If the 5th string works, I’ll use it, if not, I just don’t use it. Sometimes, playing three finger rolls on just 4 strings, hitting the 4th string in place of the 5th sounds great. Above the 5th fret, I use thumbovers and bars to bring the 5th string back into play.

…The above is how I played for over 20 years. Avoiding spikes, for some reason, thinking it would complicate things. Then I bought a used banjo with spikes, and everything changed. First and foremost, they are easy to use, and tweaking tuning slightly is easy and takes a second or two. It only took a few weeks, and I could get it almost perfect, just by repetition. Now, I spike the string, estimate the 5th tuner knob tweek, and hit a quick comparison with the 1st string same fret, and I’m ready to go. Maybe 5 seconds. Now, all my banjos have spikes at 7, 9, and 10. I would never go back to my old way… but still use some of those techniques from time to time.

And, thanks to a recent thread here, I occasionally retune my 5th string to g# for playing in E, and really like it. I did add a spike to the 6th fret on one of my banjos for this.

Have you checked out "alternatives" to spikes and retuning? There are a few low-buck options that may get you by. I personally use the small 3d printed 5th string capo as seen here being sold in BHO classifieds: (no relation to the seller, linking to it only for illustrative purposes) https://www.banjohangout.org/classified/111352

It's a little nub you put under the 5th string at the desired fret. I own a 3d printer and make them myself, but you can also make one out of a pen cap or wood if you don't have access to a 3d printer. The down side is that they're easy to lose, but they work!