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 $139.29   3.540%   $144.2209   $144.22   $144.20   $144.20  
  Purchase #2 $106.33   6.266%   $112.9926   $112.99   $113.00   $113.00  
  Purchase #3 $66.74   7.181%   $71.5326   $71.53   $71.55   $71.50  
  Purchase #4 $65.98   8.831%   $71.8067   $71.81   $71.80   $71.80  
  Purchase #5 $125.56   12.598%   $141.378   $141.38   $141.40   $141.40  
  Purchase #6 $75.45   13.675%   $85.7678   $85.77   $85.75   $85.80  
  Purchase #7 $14.67   10.349%   $16.1882   $16.19   $16.20   $16.20  
  7 purchase totals = $594.02       $643.8868   $643.89   $643.90   $643.90