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 $124.47   12.374%   $139.8719   $139.87   $139.85   $139.90  
  Purchase #2 $146.33   11.665%   $163.3994   $163.40   $163.40   $163.40  
  Purchase #3 $224.82   7.198%   $241.0025   $241.00   $241.00   $241.00  
  Purchase #4 $143.31   11.267%   $159.4567   $159.46   $159.45   $159.50  
  Purchase #5 $84.63   8.157%   $91.5333   $91.53   $91.55   $91.50  
  Purchase #6 $141.86   4.310%   $147.9742   $147.97   $147.95   $148.00  
  Purchase #7 $86.80   8.704%   $94.3551   $94.36   $94.35   $94.40  
  7 purchase totals = $952.22       $1,037.5931   $1,037.59   $1,037.55   $1,037.70