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 $17.62   4.587%   $18.4282   $18.43   $18.45   $18.40  
  Purchase #2 $30.47   12.307%   $34.2199   $34.22   $34.20   $34.20  
  Purchase #3 $233.41   13.241%   $264.3158   $264.32   $264.30   $264.30  
  Purchase #4 $25.38   10.145%   $27.9548   $27.95   $27.95   $28.00  
  Purchase #5 $48.44   7.501%   $52.0735   $52.07   $52.05   $52.10  
  Purchase #6 $122.43   4.357%   $127.7643   $127.76   $127.75   $127.80  
  Purchase #7 $15.53   7.306%   $16.6646   $16.66   $16.65   $16.70  
  7 purchase totals = $493.28       $541.4211   $541.41   $541.35   $541.50