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 $244.49   12.848%   $275.9021   $275.90   $275.90   $275.90  
  Purchase #2 $121.38   10.754%   $134.4332   $134.43   $134.45   $134.40  
  Purchase #3 $231.60   3.439%   $239.5647   $239.56   $239.55   $239.60  
  Purchase #4 $250.86   5.125%   $263.7166   $263.72   $263.70   $263.70  
  Purchase #5 $42.89   11.264%   $47.7211   $47.72   $47.70   $47.70  
  Purchase #6 $200.34   3.188%   $206.7268   $206.73   $206.75   $206.70  
  Purchase #7 $59.99   4.694%   $62.8059   $62.81   $62.80   $62.80  
  7 purchase totals = $1,151.55       $1,230.8704   $1,230.87   $1,230.85   $1,230.80