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 $238.55   6.202%   $253.3449   $253.34   $253.35   $253.30  
  Purchase #2 $18.86   9.711%   $20.6915   $20.69   $20.70   $20.70  
  Purchase #3 $88.29   4.546%   $92.3037   $92.30   $92.30   $92.30  
  Purchase #4 $73.58   4.859%   $77.1553   $77.16   $77.15   $77.20  
  Purchase #5 $240.51   8.362%   $260.6214   $260.62   $260.60   $260.60  
  Purchase #6 $246.62   9.830%   $270.8627   $270.86   $270.85   $270.90  
  Purchase #7 $38.68   8.596%   $42.0049   $42.00   $42.00   $42.00  
  7 purchase totals = $945.09       $1,016.9844   $1,016.97   $1,016.95   $1,017.00