我experience was 我七月中去,正正在日宣佈量寬前exchange, 7.64 換。first day arrival, starts to go down.....almost 7.2 during my vacation. so I have used credit card sever times to recover the exch lost.
Because I believe jpy is undergoing upward trend. there are always slumps in the course, but they don't change the overall trend.
so it depends on what you have believed to be about jpy. as I believe it's on the up, so exchange some/use credit cards at drops. never know the bottom, once you think you know it, it's gone.