It doesn't hurt anything to both force write the database and erase the favorites prior to writing if you are making all edits/changes in Sentinel which is what I do, so I check both boxes each time. If you use Zip Code scanning and/or location scanning, you definitely want to update the database. If all you are scanning is favorites, then it is not using the database, it is using whatever info that is in the favorites. The best reason for erasing (or clearing) favorites in scanner before writing is that sometimes if you use other software packages to also edit stuff, you can wind up with duplicate favorites, I.e. Coffey County and then another Coffee County out there. It is not always obvious which one Is going to get scanned.
However if you go to software like ProScan which a lot people use and you edit all of your favorites with ProScan, you don't want to erase the scanner unless you have read the info from the scanner into Sentinel, then made changes in Sentinel and write back to the scanner. But then you have to read the scanner info back into ProScan before editing stuff there. Some people learn that the hard way when things disappear. ProScan can not edit Profiles, so they have to be done in Sentinel.
Also keep in mind if you change stuff in the scanner itself and don't read from the scanner into Sentinel before you do edits in Sentinel, those changes will likely get overwritten no matter which box you check.
Sentinel is suppose to be good at looking the date of file containing the edit and other info and then deciding whether to replace the favorites file when writing to the scanner. In other words, if you have 4 favorites but only edit 1 of them, it is suppose only write back the one that was edited and leave the other 3 alone. That seems to work most times.