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 $249.98   9.419%   $273.5256   $273.53   $273.55   $273.50  
  Purchase #2 $141.39   6.276%   $150.2636   $150.26   $150.25   $150.30  
  Purchase #3 $17.67   5.716%   $18.68   $18.68   $18.70   $18.70  
  Purchase #4 $219.62   6.618%   $234.1545   $234.15   $234.15   $234.20  
  Purchase #5 $124.36   10.850%   $137.8531   $137.85   $137.85   $137.90  
  Purchase #6 $51.81   7.193%   $55.5367   $55.54   $55.55   $55.50  
  Purchase #7 $134.54   5.362%   $141.754   $141.75   $141.75   $141.80  
  7 purchase totals = $939.37       $1,011.7675   $1,011.76   $1,011.80   $1,011.90