I bought my tacoma with 143K on it and i didn't know if it ever had a transmission flush. I asked around for advice from several people and this is what they told me:
1) Don't ever change it unless something goes wrong but makes sure the fluid is full always.
2) Never flush it but do drain it because flushing it can cause more problems if it was never flushed with regular maintenance.
i have 153K on my truck now.