Well strings spacing is one thing that really can influence your playing especially for some technique as the slap.
It's true that when you're playing always and only the same instrument you get used even on narrow strings spacing but it's also true that it depends from what kind of lines you have to play

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The fact is that if you have to slap a particular line that require a lot of plucking on "inside" (as the example attached with double stops) strings as A or D (or G if you play a six strings) merged with complex rhythm, then is another story and narrow spacing could be very uncomfortable..
That's why many players usually, play instruments with the same strings spacing, if you wanna be versatile and be able to play the same thing you do on a regular four strings bass then you always need the 19mm strings spacing, several bassist feel comfortable to use also the 18mm on 5 and 6 strings instruments, others as me prefer just always use the 19, because I can even feel 1mm of difference.
If you don't wanna feel any limits on your playing when you switch basses then use always or almost the same spacing otherwise you need to get used to play different lines on different basses