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 $166.87   6.191%   $177.2009   $177.20   $177.20   $177.20  
  Purchase #2 $232.49   11.272%   $258.6963   $258.70   $258.70   $258.70  
  Purchase #3 $9.30   4.414%   $9.7105   $9.71   $9.70   $9.70  
  Purchase #4 $87.30   8.258%   $94.5092   $94.51   $94.50   $94.50  
  Purchase #5 $64.45   3.148%   $66.4789   $66.48   $66.50   $66.50  
  Purchase #6 $140.93   6.726%   $150.409   $150.41   $150.40   $150.40  
  Purchase #7 $192.75   3.763%   $200.0032   $200.00   $200.00   $200.00  
  7 purchase totals = $894.09       $957.0080   $957.01   $957.00   $957.00