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 $8.38   3.739%   $8.6933   $8.69   $8.70   $8.70  
  Purchase #2 $82.67   7.579%   $88.9356   $88.94   $88.95   $88.90  
  Purchase #3 $230.53   13.444%   $261.5225   $261.52   $261.50   $261.50  
  Purchase #4 $61.92   12.654%   $69.7554   $69.76   $69.75   $69.80  
  Purchase #5 $6.78   12.142%   $7.6032   $7.60   $7.60   $7.60  
  Purchase #6 $103.73   3.273%   $107.1251   $107.13   $107.15   $107.10  
  Purchase #7 $31.88   3.125%   $32.8763   $32.88   $32.90   $32.90  
  7 purchase totals = $525.89       $576.5114   $576.52   $576.55   $576.50