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 $16.37   10.571%   $18.1005   $18.10   $18.10   $18.10  
  Purchase #2 $86.57   7.476%   $93.042   $93.04   $93.05   $93.00  
  Purchase #3 $69.35   10.625%   $76.7184   $76.72   $76.70   $76.70  
  Purchase #4 $89.29   11.845%   $99.8664   $99.87   $99.85   $99.90  
  Purchase #5 $40.76   13.281%   $46.1733   $46.17   $46.15   $46.20  
  Purchase #6 $26.90   8.768%   $29.2586   $29.26   $29.25   $29.30  
  Purchase #7 $245.75   12.584%   $276.6752   $276.68   $276.70   $276.70  
  7 purchase totals = $574.99       $639.8344   $639.84   $639.80   $639.90