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 $13.53   13.708%   $15.3847   $15.38   $15.40   $15.40  
  Purchase #2 $186.80   9.612%   $204.7552   $204.76   $204.75   $204.80  
  Purchase #3 $23.96   6.625%   $25.5474   $25.55   $25.55   $25.50  
  Purchase #4 $57.80   12.226%   $64.8666   $64.87   $64.85   $64.90  
  Purchase #5 $94.30   7.679%   $101.5413   $101.54   $101.55   $101.50  
  Purchase #6 $91.81   13.864%   $104.5385   $104.54   $104.55   $104.50  
  Purchase #7 $116.39   5.156%   $122.3911   $122.39   $122.40   $122.40  
  7 purchase totals = $584.59       $639.0248   $639.03   $639.05   $639.00