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 $235.90   10.824%   $261.4338   $261.43   $261.45   $261.40  
  Purchase #2 $133.86   10.659%   $148.1281   $148.13   $148.15   $148.10  
  Purchase #3 $13.29   5.435%   $14.0123   $14.01   $14.00   $14.00  
  Purchase #4 $153.90   6.521%   $163.9358   $163.94   $163.95   $163.90  
  Purchase #5 $153.55   8.401%   $166.4497   $166.45   $166.45   $166.40  
  Purchase #6 $80.87   6.598%   $86.2058   $86.21   $86.20   $86.20  
  Purchase #7 $140.40   6.724%   $149.8405   $149.84   $149.85   $149.80  
  7 purchase totals = $911.77       $990.0060   $990.01   $990.05   $989.80