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 $223.29   13.106%   $252.5544   $252.55   $252.55   $252.60  
  Purchase #2 $165.47   6.765%   $176.664   $176.66   $176.65   $176.70  
  Purchase #3 $38.95   10.779%   $43.1484   $43.15   $43.15   $43.10  
  Purchase #4 $17.55   12.172%   $19.6862   $19.69   $19.70   $19.70  
  Purchase #5 $17.26   5.788%   $18.259   $18.26   $18.25   $18.30  
  Purchase #6 $45.81   6.410%   $48.7464   $48.75   $48.75   $48.70  
  Purchase #7 $111.80   7.153%   $119.7971   $119.80   $119.80   $119.80  
  7 purchase totals = $620.13       $678.8555   $678.86   $678.85   $678.90