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 $8.78   12.687%   $9.8939   $9.89   $9.90   $9.90  
  Purchase #2 $8.59   5.785%   $9.0869   $9.09   $9.10   $9.10  
  Purchase #3 $144.32   7.870%   $155.678   $155.68   $155.70   $155.70  
  Purchase #4 $247.35   3.746%   $256.6157   $256.62   $256.60   $256.60  
  Purchase #5 $37.76   7.546%   $40.6094   $40.61   $40.60   $40.60  
  Purchase #6 $222.35   3.663%   $230.4947   $230.49   $230.50   $230.50  
  Purchase #7 $36.48   9.700%   $40.0186   $40.02   $40.00   $40.00  
  7 purchase totals = $705.63       $742.3972   $742.40   $742.40   $742.40