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 $52.77   9.711%   $57.8945   $57.89   $57.90   $57.90  
  Purchase #2 $84.27   13.632%   $95.7577   $95.76   $95.75   $95.80  
  Purchase #3 $210.54   4.329%   $219.6543   $219.65   $219.65   $219.70  
  Purchase #4 $55.71   5.695%   $58.8827   $58.88   $58.90   $58.90  
  Purchase #5 $200.45   13.524%   $227.5589   $227.56   $227.55   $227.60  
  Purchase #6 $232.88   8.817%   $253.413   $253.41   $253.40   $253.40  
  Purchase #7 $50.75   13.254%   $57.4764   $57.48   $57.50   $57.50  
  7 purchase totals = $887.37       $970.6375   $970.63   $970.65   $970.80