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 $90.57   12.804%   $102.1666   $102.17   $102.15   $102.20  
  Purchase #2 $156.28   9.808%   $171.6079   $171.61   $171.60   $171.60  
  Purchase #3 $62.77   3.284%   $64.8314   $64.83   $64.85   $64.80  
  Purchase #4 $145.34   12.255%   $163.1514   $163.15   $163.15   $163.20  
  Purchase #5 $168.28   8.752%   $183.0079   $183.01   $183.00   $183.00  
  Purchase #6 $137.50   5.606%   $145.2083   $145.21   $145.20   $145.20  
  Purchase #7 $142.82   11.700%   $159.5299   $159.53   $159.55   $159.50  
  7 purchase totals = $903.56       $989.5034   $989.51   $989.50   $989.50