For many years, the standard guidance -- from the owner's manual -- was to not drive the vehicle faster than 55 (or even 45) until the engine was broken in. I remember distinctly from one of my Camry manuals (95 or 01, don't remember which) the instruction to "not drive at a constant speed, but to vary the speed a little from time to time" -- all in the name of conditioning the valves and pistons, etc. Could be the sales person is remembering the rules of yore -- no, that doesn't excuse inaccurate information -- or just being extra cautionary. It certainly won't HURT to drive very conservatively for a short while. I'm not an engineer, and couldn't possible argue the merits of any of these approaches, but flyingn is right that these warnings of old do not appear anymore in the manual.
I also remember warnings not to use synthetic oil until after a LONG break-in, but some manufacturers today use synthetic as original factory oil fill, so ---- I'm not too worried about that, either, these days. One thing I will recommend against is the habitual use of engine cleaners that you put into the crankcase while idling the engine for 5 minutes. Yeah, they might give you a nice, clean engine, but they will definitely wear out your seals prematurely (it happened to me), precipitating some expensive repairs down the road. Besides, if you use synthetic oil regularly, your engine will stay clean anyway.
Sorry for the off-topic, but it's all about caring for our engines.