From the posted Rulebook changes page (http://www.uschess.org/docs/gov/reports/RulebookChanges.pdf):
15B. Scorekeeping in time pressure, non-sudden death time control. If either player has less than five
minutes remaining in a non-sudden death time control and does not have additional time (increment) of 30
seconds or more added with each move, both players are excused from the obligation to keep score…
TD TIP: Only players in games with increment time controls of 30 seconds or more and using
properly set increment capable clocks are required to keep score at all times, even in the last five
minutes of any time control period. Players using improperly set increment clocks or non-increment
capable clocks, even those clocks adjusted for an increment time control, are regulated by rule 15B.
15C. Scorekeeping in time pressure, sudden death time control. If Either Player has less than five
minutes remaining in a sudden death time control and does not have additional time (increment) of 30
seconds or more added with each move, both players are excused from the obligation to keep score….
TD TIP: Only players in games with increment time controls of 30 seconds or more and using
properly set increment capable clocks are required to keep score at all times, even in the last five
minutes of any time control period. Players using improperly set increment clocks or non-increment
capable clocks, even those clocks adjusted for an increment time control, are regulated by rule 15C.
From the rulebook:
15B. Scorekeeping in time pressure, non-sudden death time control.
If either player has less than five minutes remaining in a non-sudden-death time control, both players are excused from the obligation to keep score until the end of the time control period. Doing so, however, may make it impossible to claim a draw by triple occurrence of position (14C) or the 50-move rule (14F) or a win on time forfeit (13C). Scorekeeping by both players must resume with the start of the next time control period, and missing moves should be filled in (15F).
15C. Scorekeeping in time pressure, sudden death time control.
If either player has less than five minutes remaining in a sudden-death time control, both players are excused from the obligation to keep score. A scoresheet is not required to win on time in a sudden death control (13C).