Yes, the right time to buy TIPS is when the fixed rate is high, the problem being guessing what "high" is relative to other options at a given time.

Just to make sure I'm not getting confused, isn't the time to buy TIPS as opposed to traditional bonds when the real rate offered by TIPS is higher than your own estimate of the real rate being offered by traditional bonds? For example, TIPS at 2.5% might be a great deal compared to traditional bonds at 4.5% if you think inflation will be higher than 2%, while TIPS at 3.5% might not be a good deal compared to traditional bonds at 6.5% if you think inflation will be less than 3%.

Granted, if you can buy when inflation expectations are low and then those expectations get higher, then you'll end up winning on both ends. But it's easy to focus on the rate and not look beyond it to what your other options are.

dan