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 $8.61   5.192%   $9.057   $9.06   $9.05   $9.10  
  Purchase #2 $106.58   8.180%   $115.2982   $115.30   $115.30   $115.30  
  Purchase #3 $15.68   5.526%   $16.5465   $16.55   $16.55   $16.50  
  Purchase #4 $60.79   6.479%   $64.7286   $64.73   $64.75   $64.70  
  Purchase #5 $123.95   7.663%   $133.4483   $133.45   $133.45   $133.40  
  Purchase #6 $54.67   3.498%   $56.5824   $56.58   $56.60   $56.60  
  Purchase #7 $217.33   12.439%   $244.3637   $244.36   $244.35   $244.40  
  7 purchase totals = $587.61       $640.0247   $640.03   $640.05   $640.00