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 $246.68   12.113%   $276.5603   $276.56   $276.55   $276.60  
  Purchase #2 $174.29   9.286%   $190.4746   $190.47   $190.45   $190.50  
  Purchase #3 $54.79   8.157%   $59.2592   $59.26   $59.25   $59.30  
  Purchase #4 $104.61   11.854%   $117.0105   $117.01   $117.00   $117.00  
  Purchase #5 $167.30   6.163%   $177.6107   $177.61   $177.60   $177.60  
  Purchase #6 $104.49   4.405%   $109.0928   $109.09   $109.10   $109.10  
  Purchase #7 $45.81   9.587%   $50.2018   $50.20   $50.20   $50.20  
  7 purchase totals = $897.97       $980.2099   $980.20   $980.15   $980.30