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 $6.33   7.300%   $6.7921   $6.79   $6.80   $6.80  
  Purchase #2 $17.70   10.762%   $19.6049   $19.60   $19.60   $19.60  
  Purchase #3 $18.44   4.180%   $19.2108   $19.21   $19.20   $19.20  
  Purchase #4 $177.43   10.405%   $195.8916   $195.89   $195.90   $195.90  
  Purchase #5 $117.73   8.802%   $128.0926   $128.09   $128.10   $128.10  
  Purchase #6 $163.89   3.283%   $169.2705   $169.27   $169.25   $169.30  
  Purchase #7 $93.91   6.843%   $100.3363   $100.34   $100.35   $100.30  
  7 purchase totals = $595.43       $639.1988   $639.19   $639.20   $639.20