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 $83.57   6.538%   $89.0338   $89.03   $89.05   $89.00  
  Purchase #2 $121.50   12.855%   $137.1188   $137.12   $137.10   $137.10  
  Purchase #3 $120.48   7.542%   $129.5666   $129.57   $129.55   $129.60  
  Purchase #4 $113.90   4.413%   $118.9264   $118.93   $118.95   $118.90  
  Purchase #5 $65.56   5.359%   $69.0734   $69.07   $69.05   $69.10  
  Purchase #6 $77.55   7.525%   $83.3856   $83.39   $83.40   $83.40  
  Purchase #7 $108.86   12.726%   $122.7135   $122.71   $122.70   $122.70  
  7 purchase totals = $691.42       $749.8181   $749.82   $749.80   $749.80