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 $17.35   4.266%   $18.0902   $18.09   $18.10   $18.10  
  Purchase #2 $60.68   6.155%   $64.4149   $64.41   $64.40   $64.40  
  Purchase #3 $71.51   6.560%   $76.2011   $76.20   $76.20   $76.20  
  Purchase #4 $16.52   10.358%   $18.2311   $18.23   $18.25   $18.20  
  Purchase #5 $166.44   6.450%   $177.1754   $177.18   $177.20   $177.20  
  Purchase #6 $163.66   9.467%   $179.1537   $179.15   $179.15   $179.20  
  Purchase #7 $85.69   12.691%   $96.5649   $96.56   $96.55   $96.60  
  7 purchase totals = $581.85       $629.8313   $629.82   $629.85   $629.90