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 $82.99   6.151%   $88.0947   $88.09   $88.10   $88.10  
  Purchase #2 $204.93   12.701%   $230.9582   $230.96   $230.95   $231.00  
  Purchase #3 $127.58   3.626%   $132.2061   $132.21   $132.20   $132.20  
  Purchase #4 $227.91   11.235%   $253.5157   $253.52   $253.50   $253.50  
  Purchase #5 $202.82   12.654%   $228.4848   $228.48   $228.50   $228.50  
  Purchase #6 $241.28   8.306%   $261.3207   $261.32   $261.30   $261.30  
  Purchase #7 $133.80   6.138%   $142.0126   $142.01   $142.00   $142.00  
  7 purchase totals = $1,221.31       $1,336.5928   $1,336.59   $1,336.55   $1,336.60