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 $18.99   10.556%   $20.9946   $20.99   $21.00   $21.00  
  Purchase #2 $65.72   3.410%   $67.9611   $67.96   $67.95   $68.00  
  Purchase #3 $191.27   9.582%   $209.5975   $209.60   $209.60   $209.60  
  Purchase #4 $209.63   4.716%   $219.5162   $219.52   $219.50   $219.50  
  Purchase #5 $71.30   13.612%   $81.0054   $81.01   $81.00   $81.00  
  Purchase #6 $161.96   6.318%   $172.1926   $172.19   $172.20   $172.20  
  Purchase #7 $120.65   11.843%   $134.9386   $134.94   $134.95   $134.90  
  7 purchase totals = $839.52       $906.2060   $906.21   $906.20   $906.20