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 $246.85   11.793%   $275.961   $275.96   $275.95   $276.00  
  Purchase #2 $154.36   4.806%   $161.7785   $161.78   $161.80   $161.80  
  Purchase #3 $89.91   3.398%   $92.9651   $92.97   $92.95   $93.00  
  Purchase #4 $166.74   5.397%   $175.739   $175.74   $175.75   $175.70  
  Purchase #5 $161.94   4.801%   $169.7147   $169.71   $169.70   $169.70  
  Purchase #6 $65.27   7.147%   $69.9348   $69.93   $69.95   $69.90  
  Purchase #7 $41.58   3.126%   $42.8798   $42.88   $42.90   $42.90  
  7 purchase totals = $926.65       $988.9729   $988.97   $989.00   $989.00