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 $135.40   6.430%   $144.1062   $144.11   $144.10   $144.10  
  Purchase #2 $239.80   8.589%   $260.3964   $260.40   $260.40   $260.40  
  Purchase #3 $34.50   13.288%   $39.0844   $39.08   $39.10   $39.10  
  Purchase #4 $73.59   10.111%   $81.0307   $81.03   $81.05   $81.00  
  Purchase #5 $132.60   8.166%   $143.4281   $143.43   $143.45   $143.40  
  Purchase #6 $86.74   8.782%   $94.3575   $94.36   $94.35   $94.40  
  Purchase #7 $133.53   7.656%   $143.7531   $143.75   $143.75   $143.80  
  7 purchase totals = $836.16       $906.1564   $906.16   $906.20   $906.20