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 $235.61   12.195%   $264.3426   $264.34   $264.35   $264.30  
  Purchase #2 $138.25   13.671%   $157.1502   $157.15   $157.15   $157.20  
  Purchase #3 $160.33   5.793%   $169.6179   $169.62   $169.60   $169.60  
  Purchase #4 $196.79   3.713%   $204.0968   $204.10   $204.10   $204.10  
  Purchase #5 $9.49   3.801%   $9.8507   $9.85   $9.85   $9.90  
  Purchase #6 $119.30   12.769%   $134.5334   $134.53   $134.55   $134.50  
  Purchase #7 $77.86   12.451%   $87.5543   $87.55   $87.55   $87.60  
  7 purchase totals = $937.63       $1,027.1459   $1,027.14   $1,027.15   $1,027.20