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 $81.70   11.229%   $90.8741   $90.87   $90.85   $90.90  
  Purchase #2 $124.70   9.644%   $136.7261   $136.73   $136.75   $136.70  
  Purchase #3 $7.52   5.721%   $7.9502   $7.95   $7.95   $8.00  
  Purchase #4 $166.94   3.282%   $172.419   $172.42   $172.40   $172.40  
  Purchase #5 $80.36   3.293%   $83.0063   $83.01   $83.00   $83.00  
  Purchase #6 $78.50   13.156%   $88.8275   $88.83   $88.85   $88.80  
  Purchase #7 $84.35   5.447%   $88.9445   $88.94   $88.95   $88.90  
  7 purchase totals = $624.07       $668.7477   $668.75   $668.75   $668.70