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 $19.62   7.616%   $21.1143   $21.11   $21.10   $21.10  
  Purchase #2 $12.88   3.559%   $13.3384   $13.34   $13.35   $13.30  
  Purchase #3 $160.91   7.839%   $173.5237   $173.52   $173.50   $173.50  
  Purchase #4 $115.63   4.814%   $121.1964   $121.20   $121.20   $121.20  
  Purchase #5 $37.51   4.856%   $39.3315   $39.33   $39.35   $39.30  
  Purchase #6 $215.54   7.361%   $231.4059   $231.41   $231.40   $231.40  
  Purchase #7 $208.39   11.423%   $232.1944   $232.19   $232.20   $232.20  
  7 purchase totals = $770.48       $832.1046   $832.10   $832.10   $832.00