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 $18.37   7.141%   $19.6818   $19.68   $19.70   $19.70  
  Purchase #2 $120.89   7.332%   $129.7537   $129.75   $129.75   $129.80  
  Purchase #3 $231.43   11.142%   $257.2159   $257.22   $257.20   $257.20  
  Purchase #4 $9.48   8.294%   $10.2663   $10.27   $10.25   $10.30  
  Purchase #5 $43.26   11.430%   $48.2046   $48.20   $48.20   $48.20  
  Purchase #6 $32.78   10.462%   $36.2094   $36.21   $36.20   $36.20  
  Purchase #7 $147.42   11.733%   $164.7168   $164.72   $164.70   $164.70  
  7 purchase totals = $603.63       $666.0485   $666.05   $666.00   $666.10