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 $17.90   3.597%   $18.5439   $18.54   $18.55   $18.50  
  Purchase #2 $149.61   5.589%   $157.9717   $157.97   $157.95   $158.00  
  Purchase #3 $238.90   5.830%   $252.8279   $252.83   $252.85   $252.80  
  Purchase #4 $191.67   6.323%   $203.7893   $203.79   $203.80   $203.80  
  Purchase #5 $206.77   7.546%   $222.3729   $222.37   $222.35   $222.40  
  Purchase #6 $208.48   13.560%   $236.7499   $236.75   $236.75   $236.70  
  Purchase #7 $158.57   12.134%   $177.8109   $177.81   $177.80   $177.80  
  7 purchase totals = $1,171.90       $1,270.0665   $1,270.06   $1,270.05   $1,270.00