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 $67.93   11.212%   $75.5463   $75.55   $75.55   $75.50  
  Purchase #2 $188.38   8.414%   $204.2303   $204.23   $204.25   $204.20  
  Purchase #3 $95.28   6.746%   $101.7076   $101.71   $101.70   $101.70  
  Purchase #4 $235.80   6.198%   $250.4149   $250.41   $250.40   $250.40  
  Purchase #5 $69.74   8.791%   $75.8708   $75.87   $75.85   $75.90  
  Purchase #6 $12.81   4.813%   $13.4265   $13.43   $13.45   $13.40  
  Purchase #7 $131.92   3.556%   $136.6111   $136.61   $136.60   $136.60  
  7 purchase totals = $801.86       $857.8075   $857.81   $857.80   $857.70