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 $8.41   9.839%   $9.2375   $9.24   $9.25   $9.20  
  Purchase #2 $46.62   7.454%   $50.0951   $50.10   $50.10   $50.10  
  Purchase #3 $129.96   7.387%   $139.5601   $139.56   $139.55   $139.60  
  Purchase #4 $179.65   13.121%   $203.2219   $203.22   $203.20   $203.20  
  Purchase #5 $130.94   5.171%   $137.7109   $137.71   $137.70   $137.70  
  Purchase #6 $76.87   4.192%   $80.0924   $80.09   $80.10   $80.10  
  Purchase #7 $18.68   8.656%   $20.2969   $20.30   $20.30   $20.30  
  7 purchase totals = $591.13       $640.2148   $640.22   $640.20   $640.20