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 $20.43   4.625%   $21.3749   $21.37   $21.35   $21.40  
  Purchase #2 $10.36   8.260%   $11.2157   $11.22   $11.20   $11.20  
  Purchase #3 $100.92   3.540%   $104.4926   $104.49   $104.50   $104.50  
  Purchase #4 $80.47   5.739%   $85.0882   $85.09   $85.10   $85.10  
  Purchase #5 $63.65   8.716%   $69.1977   $69.20   $69.20   $69.20  
  Purchase #6 $248.66   12.640%   $280.0906   $280.09   $280.10   $280.10  
  Purchase #7 $73.98   3.419%   $76.5094   $76.51   $76.50   $76.50  
  7 purchase totals = $598.47       $647.9691   $647.97   $647.95   $648.00