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 $231.69   12.365%   $260.3385   $260.34   $260.35   $260.30  
  Purchase #2 $207.47   8.637%   $225.3892   $225.39   $225.40   $225.40  
  Purchase #3 $185.98   8.782%   $202.3128   $202.31   $202.30   $202.30  
  Purchase #4 $60.25   11.217%   $67.0082   $67.01   $67.00   $67.00  
  Purchase #5 $59.74   13.490%   $67.7989   $67.80   $67.80   $67.80  
  Purchase #6 $241.32   10.621%   $266.9506   $266.95   $266.95   $267.00  
  Purchase #7 $164.39   11.156%   $182.7293   $182.73   $182.75   $182.70  
  7 purchase totals = $1,150.84       $1,272.5275   $1,272.53   $1,272.55   $1,272.50