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 $78.95   12.617%   $88.9111   $88.91   $88.90   $88.90  
  Purchase #2 $131.68   5.299%   $138.6577   $138.66   $138.65   $138.70  
  Purchase #3 $230.56   4.143%   $240.1121   $240.11   $240.10   $240.10  
  Purchase #4 $151.66   5.266%   $159.6464   $159.65   $159.65   $159.60  
  Purchase #5 $108.94   6.491%   $116.0113   $116.01   $116.00   $116.00  
  Purchase #6 $205.28   9.143%   $224.0488   $224.05   $224.05   $224.00  
  Purchase #7 $42.38   5.272%   $44.6143   $44.61   $44.60   $44.60  
  7 purchase totals = $949.45       $1,012.0017   $1,012.00   $1,011.95   $1,011.90