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 $15.80   12.450%   $17.7671   $17.77   $17.75   $17.80  
  Purchase #2 $56.31   12.770%   $63.5008   $63.50   $63.50   $63.50  
  Purchase #3 $28.40   10.787%   $31.4635   $31.46   $31.45   $31.50  
  Purchase #4 $146.42   12.677%   $164.9817   $164.98   $165.00   $165.00  
  Purchase #5 $157.29   12.391%   $176.7798   $176.78   $176.80   $176.80  
  Purchase #6 $164.84   13.401%   $186.9302   $186.93   $186.95   $186.90  
  Purchase #7 $123.76   7.817%   $133.4343   $133.43   $133.45   $133.40  
  7 purchase totals = $692.82       $774.8574   $774.85   $774.90   $774.90