Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $240.88   13.648%   $273.7553   $273.76   $273.75   $273.80  
  Purchase #2 $61.29   7.213%   $65.7108   $65.71   $65.70   $65.70  
  Purchase #3 $250.86   11.234%   $279.0416   $279.04   $279.05   $279.00  
  Purchase #4 $206.31   11.832%   $230.7206   $230.72   $230.70   $230.70  
  Purchase #5 $44.49   6.418%   $47.3454   $47.35   $47.35   $47.30  
  Purchase #6 $116.53   6.673%   $124.306   $124.31   $124.30   $124.30  
  Purchase #7 $195.80   3.587%   $202.8233   $202.82   $202.80   $202.80  
  7 purchase totals = $1,116.16       $1,223.7030   $1,223.71   $1,223.65   $1,223.60