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 $18.44   10.223%   $20.3251   $20.33   $20.35   $20.30  
  Purchase #2 $71.54   4.166%   $74.5204   $74.52   $74.50   $74.50  
  Purchase #3 $49.31   8.843%   $53.6705   $53.67   $53.65   $53.70  
  Purchase #4 $21.36   4.687%   $22.3611   $22.36   $22.35   $22.40  
  Purchase #5 $18.40   7.335%   $19.7496   $19.75   $19.75   $19.70  
  Purchase #6 $17.80   9.205%   $19.4385   $19.44   $19.45   $19.40  
  Purchase #7 $178.77   10.411%   $197.3817   $197.38   $197.40   $197.40  
  7 purchase totals = $375.62       $407.4469   $407.45   $407.45   $407.40