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 $18.57   7.182%   $19.9037   $19.90   $19.90   $19.90  
  Purchase #2 $16.70   7.780%   $17.9993   $18.00   $18.00   $18.00  
  Purchase #3 $172.72   10.241%   $190.4083   $190.41   $190.40   $190.40  
  Purchase #4 $175.26   13.350%   $198.6572   $198.66   $198.65   $198.70  
  Purchase #5 $124.39   7.137%   $133.2677   $133.27   $133.25   $133.30  
  Purchase #6 $72.48   7.637%   $78.0153   $78.02   $78.00   $78.00  
  Purchase #7 $156.91   10.625%   $173.5817   $173.58   $173.60   $173.60  
  7 purchase totals = $737.03       $811.8332   $811.84   $811.80   $811.90