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 $6.30   9.414%   $6.8931   $6.89   $6.90   $6.90  
  Purchase #2 $104.59   3.398%   $108.144   $108.14   $108.15   $108.10  
  Purchase #3 $26.97   3.317%   $27.8646   $27.86   $27.85   $27.90  
  Purchase #4 $188.87   4.856%   $198.0415   $198.04   $198.05   $198.00  
  Purchase #5 $203.86   12.766%   $229.8848   $229.88   $229.90   $229.90  
  Purchase #6 $22.30   8.549%   $24.2064   $24.21   $24.20   $24.20  
  Purchase #7 $47.57   8.682%   $51.7   $51.70   $51.70   $51.70  
  7 purchase totals = $600.46       $646.7344   $646.72   $646.75   $646.70