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 $197.73   10.450%   $218.3928   $218.39   $218.40   $218.40  
  Purchase #2 $16.26   9.423%   $17.7922   $17.79   $17.80   $17.80  
  Purchase #3 $9.89   12.838%   $11.1597   $11.16   $11.15   $11.20  
  Purchase #4 $101.80   10.772%   $112.7659   $112.77   $112.75   $112.80  
  Purchase #5 $237.82   8.258%   $257.4592   $257.46   $257.45   $257.50  
  Purchase #6 $5.52   4.418%   $5.7639   $5.76   $5.75   $5.80  
  Purchase #7 $15.34   9.135%   $16.7413   $16.74   $16.75   $16.70  
  7 purchase totals = $584.36       $640.0750   $640.07   $640.05   $640.20