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 $13.96   6.675%   $14.8918   $14.89   $14.90   $14.90  
  Purchase #2 $73.75   5.764%   $78.001   $78.00   $78.00   $78.00  
  Purchase #3 $187.65   10.356%   $207.083   $207.08   $207.10   $207.10  
  Purchase #4 $171.86   6.722%   $183.4124   $183.41   $183.40   $183.40  
  Purchase #5 $90.46   13.445%   $102.6223   $102.62   $102.60   $102.60  
  Purchase #6 $148.88   3.207%   $153.6546   $153.65   $153.65   $153.70  
  Purchase #7 $69.36   12.376%   $77.944   $77.94   $77.95   $77.90  
  7 purchase totals = $755.92       $817.6091   $817.59   $817.60   $817.60