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 $211.78   10.743%   $234.5315   $234.53   $234.55   $234.50  
  Purchase #2 $245.49   12.687%   $276.6353   $276.64   $276.65   $276.60  
  Purchase #3 $221.86   6.328%   $235.8993   $235.90   $235.90   $235.90  
  Purchase #4 $16.34   4.228%   $17.0309   $17.03   $17.05   $17.00  
  Purchase #5 $124.66   8.179%   $134.8559   $134.86   $134.85   $134.90  
  Purchase #6 $26.67   7.319%   $28.622   $28.62   $28.60   $28.60  
  Purchase #7 $56.86   13.618%   $64.6032   $64.60   $64.60   $64.60  
  7 purchase totals = $903.66       $992.1781   $992.18   $992.20   $992.10