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 $223.55   7.354%   $239.9899   $239.99   $240.00   $240.00  
  Purchase #2 $114.60   12.322%   $128.721   $128.72   $128.70   $128.70  
  Purchase #3 $173.35   4.583%   $181.2946   $181.29   $181.30   $181.30  
  Purchase #4 $36.52   5.390%   $38.4884   $38.49   $38.50   $38.50  
  Purchase #5 $122.74   4.491%   $128.2523   $128.25   $128.25   $128.30  
  Purchase #6 $123.56   3.638%   $128.0551   $128.06   $128.05   $128.10  
  Purchase #7 $133.84   6.255%   $142.2117   $142.21   $142.20   $142.20  
  7 purchase totals = $928.16       $987.0130   $987.01   $987.00   $987.10