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 $189.38   6.245%   $201.2068   $201.21   $201.20   $201.20  
  Purchase #2 $65.76   12.733%   $74.1332   $74.13   $74.15   $74.10  
  Purchase #3 $249.80   8.362%   $270.6883   $270.69   $270.70   $270.70  
  Purchase #4 $34.56   6.539%   $36.8199   $36.82   $36.80   $36.80  
  Purchase #5 $21.88   8.742%   $23.7927   $23.79   $23.80   $23.80  
  Purchase #6 $25.62   10.646%   $28.3475   $28.35   $28.35   $28.30  
  Purchase #7 $6.89   10.858%   $7.6381   $7.64   $7.65   $7.60  
  7 purchase totals = $593.89       $642.6265   $642.63   $642.65   $642.50