It can be a bit tricky when you're trying to credit SQ flights selectively. Once every few years I credit to LH M&M to keep up my Senator status, and at some stations they seem to have difficulty making the M&M number 'stick', requiring a redo (because boarding pass show wrong FF acct).
I don't change my default account information because I sometimes choose to credit each sector of a RT ticket to dif accounts. Just make sure that your boarding pass is printed with the right account reference as Guy Betsy wrote, be persistent if its wrong, and you'll be fine.