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 $85.29   5.358%   $89.8598   $89.86   $89.85   $89.90  
  Purchase #2 $17.61   13.563%   $19.9984   $20.00   $20.00   $20.00  
  Purchase #3 $33.48   7.139%   $35.8701   $35.87   $35.85   $35.90  
  Purchase #4 $138.80   5.337%   $146.2078   $146.21   $146.20   $146.20  
  Purchase #5 $165.46   10.272%   $182.4561   $182.46   $182.45   $182.50  
  Purchase #6 $14.66   13.662%   $16.6628   $16.66   $16.65   $16.70  
  Purchase #7 $172.47   4.828%   $180.7969   $180.80   $180.80   $180.80  
  7 purchase totals = $627.77       $671.8519   $671.86   $671.80   $672.00