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 $20.35   3.554%   $21.0732   $21.07   $21.05   $21.10  
  Purchase #2 $241.28   8.705%   $262.2834   $262.28   $262.30   $262.30  
  Purchase #3 $143.46   4.408%   $149.7837   $149.78   $149.80   $149.80  
  Purchase #4 $230.38   10.235%   $253.9594   $253.96   $253.95   $254.00  
  Purchase #5 $93.89   13.471%   $106.5379   $106.54   $106.55   $106.50  
  Purchase #6 $246.92   9.518%   $270.4218   $270.42   $270.40   $270.40  
  Purchase #7 $66.64   11.654%   $74.4062   $74.41   $74.40   $74.40  
  7 purchase totals = $1,042.92       $1,138.4656   $1,138.46   $1,138.45   $1,138.50