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 $128.49   8.848%   $139.8588   $139.86   $139.85   $139.90  
  Purchase #2 $17.29   6.410%   $18.3983   $18.40   $18.40   $18.40  
  Purchase #3 $28.26   12.344%   $31.7484   $31.75   $31.75   $31.70  
  Purchase #4 $28.38   3.503%   $29.3742   $29.37   $29.35   $29.40  
  Purchase #5 $212.40   9.531%   $232.6438   $232.64   $232.65   $232.60  
  Purchase #6 $33.68   7.414%   $36.177   $36.18   $36.20   $36.20  
  Purchase #7 $45.77   11.104%   $50.8523   $50.85   $50.85   $50.90  
  7 purchase totals = $494.27       $539.0528   $539.05   $539.05   $539.10