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 $204.61   13.139%   $231.4937   $231.49   $231.50   $231.50  
  Purchase #2 $136.99   12.123%   $153.5973   $153.60   $153.60   $153.60  
  Purchase #3 $131.62   5.547%   $138.921   $138.92   $138.90   $138.90  
  Purchase #4 $195.40   6.381%   $207.8685   $207.87   $207.85   $207.90  
  Purchase #5 $209.30   6.129%   $222.128   $222.13   $222.15   $222.10  
  Purchase #6 $218.27   7.654%   $234.9764   $234.98   $235.00   $235.00  
  Purchase #7 $115.35   8.407%   $125.0475   $125.05   $125.05   $125.00  
  7 purchase totals = $1,211.54       $1,314.0324   $1,314.04   $1,314.05   $1,314.00