Drove over 1000 miles over the last two days using Trip Planner. Each time, we always stayed a few extra minutes to add more than was recommended.
The Trip Planner seems to assume you'll be driving at the posted speed limit - so if you plan to drive above that (and stay on pace with most of the other cars on the highway), you'll need more charge than Trip Planner projects.
You'll also need more charge if conditions are not ideal - such as higher or lower temperatures, rain, high winds, traffic congestion (requiring more slowdowns), ...
Before leaving a Supercharger, we would look at two items before deciding it was time to go. We'd look at the projected energy at the next destination (PEND) - the TP usually wanted to stop when the PEND was at or approaching 10% - which really doesn't provide much cushion. We would usually stay long enough to get at least 20% for PEND.
The other number was the rated range on the dashboard. We'd use a rough estimate which we were calling our "30-20" rule. We'd take the distance to the next charger, add 30% plus 20 additional miles - and we wouldn't leave the charger until the rated range hit that spot.
And, if it looked like 30-20 was underestimating, we increased it to 40-20 for the next stop (we had a stretch with either high winds or uphill driving, which used more energy than expected).
While driving, we would monitor both the rated range and the PEND. If PEND dropped to 10%, we would slow down 5MPH. If PEND started increasing again, when it hit 15%, we'd increase speed by 5MPH. By making the adjustments early enough, we were able to ensure we would arrive at the charger with at least 25 miles of range left - keeping the battery from getting too far below 10%.
The Trip Planner seems to do a reasonable job - especially when driving at the speed limit - in ideal conditions - but probably not good enough to be counted on 100%.
We did notice some quirks in the TP software while driving. We had several periods when PEND would jump up and down, by as much as a 10% change (i.e., jumping from 11% to 21% and then back). That didn't provide us a lot of confidence in the estimates - though we could do a sanity check on the estimate by doing our own calculations using the remaining rated range and the distance left to destination.