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 $234.94   11.478%   $261.9064   $261.91   $261.90   $261.90  
  Purchase #2 $151.41   9.155%   $165.2716   $165.27   $165.25   $165.30  
  Purchase #3 $201.91   13.754%   $229.6807   $229.68   $229.70   $229.70  
  Purchase #4 $189.68   10.258%   $209.1374   $209.14   $209.15   $209.10  
  Purchase #5 $8.84   11.204%   $9.8304   $9.83   $9.85   $9.80  
  Purchase #6 $26.45   4.188%   $27.5577   $27.56   $27.55   $27.60  
  Purchase #7 $170.80   12.829%   $192.7119   $192.71   $192.70   $192.70  
  7 purchase totals = $984.03       $1,096.0961   $1,096.10   $1,096.10   $1,096.10