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 $7.89   9.514%   $8.6407   $8.64   $8.65   $8.60  
  Purchase #2 $37.38   9.249%   $40.8373   $40.84   $40.85   $40.80  
  Purchase #3 $166.41   9.322%   $181.9227   $181.92   $181.90   $181.90  
  Purchase #4 $135.46   9.377%   $148.1621   $148.16   $148.15   $148.20  
  Purchase #5 $163.43   4.369%   $170.5703   $170.57   $170.55   $170.60  
  Purchase #6 $40.29   5.666%   $42.5728   $42.57   $42.55   $42.60  
  Purchase #7 $229.57   8.765%   $249.6918   $249.69   $249.70   $249.70  
  7 purchase totals = $780.43       $842.3977   $842.39   $842.35   $842.40