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 $5.77   11.332%   $6.4239   $6.42   $6.40   $6.40  
  Purchase #2 $77.64   10.394%   $85.7099   $85.71   $85.70   $85.70  
  Purchase #3 $123.33   13.631%   $140.1411   $140.14   $140.15   $140.10  
  Purchase #4 $43.87   4.567%   $45.8735   $45.87   $45.85   $45.90  
  Purchase #5 $231.28   8.767%   $251.5563   $251.56   $251.55   $251.60  
  Purchase #6 $64.34   6.630%   $68.6057   $68.61   $68.60   $68.60  
  Purchase #7 $134.66   13.106%   $152.3085   $152.31   $152.30   $152.30  
  7 purchase totals = $680.89       $750.6189   $750.62   $750.55   $750.60