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 $28.65   10.863%   $31.7622   $31.76   $31.75   $31.80  
  Purchase #2 $25.84   5.416%   $27.2395   $27.24   $27.25   $27.20  
  Purchase #3 $28.66   9.527%   $31.3904   $31.39   $31.40   $31.40  
  Purchase #4 $238.60   9.230%   $260.6228   $260.62   $260.60   $260.60  
  Purchase #5 $57.68   10.487%   $63.7289   $63.73   $63.75   $63.70  
  Purchase #6 $180.47   8.713%   $196.1944   $196.19   $196.20   $196.20  
  Purchase #7 $34.46   9.174%   $37.6214   $37.62   $37.60   $37.60  
  7 purchase totals = $594.36       $648.5596   $648.55   $648.55   $648.50