but we all know the toilet rules, 'if it's yellow, let it mellow, if it's brown, flush it down'
I knew how to save water in the toilet, but they never taught it to me that way. Oh man you made my day, that was a pretty good one indeed
All right, back to your question.
Indeed, the same rule applies for (disk) caches; mostly for the sake of performance (analogous to the water savings in a toilet) and a little for wear issues.
Assuming a precisely timed event (i.e. triggered by a timer interrupt) of 10 Hz that writes 8 bytes to an already opened file in a SD card, means that you have at least 98 ms to spare between events; which is plenty for a cache flush (that takes at most 5 ms).
Regardless of how much data you write without flushing, this process will take the same amount of time; and as I said before, it's not enough to cause a sporadic jitter in your sampling unless the event isn't precisely timed.
Summarizing the answer of your question: "should I flush every time or should I let it happen automatically?", I will say... do whatever you feel is more convenient.
Nevertheless, keep in mind that by flushing every time, you'll be somewhat accelerating the SD card's wear; not by a panicking magnitude, but considerable enough in the long run.