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 $188.94   9.802%   $207.4599   $207.46   $207.45   $207.50  
  Purchase #2 $117.35   6.393%   $124.8522   $124.85   $124.85   $124.90  
  Purchase #3 $64.82   10.282%   $71.4848   $71.48   $71.50   $71.50  
  Purchase #4 $52.30   4.558%   $54.6838   $54.68   $54.70   $54.70  
  Purchase #5 $16.69   4.272%   $17.403   $17.40   $17.40   $17.40  
  Purchase #6 $61.68   12.612%   $69.4591   $69.46   $69.45   $69.50  
  Purchase #7 $236.83   6.265%   $251.6674   $251.67   $251.65   $251.70  
  7 purchase totals = $738.61       $797.0102   $797.00   $797.00   $797.20