I've talked to several Applications Engineers and Chemists from a couple of different paste manufacturers and they confirm that the profiles on their data sheets are nothing but guidelines. It's up to you, Process Engineer or Process Tech, to develop a thermal profile that works for the assemblies that you run in your shop.
The 248�C to 250�C that you mention is typical for lead-free, but again, just a guideline. For a leaded process, if you're reflowing SAC/RoHS BGAs, then obviously you'll want to be above 217�C and peak about 10�C higher. (Hybrid Profile).