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 $236.45   12.613%   $266.2734   $266.27   $266.25   $266.30  
  Purchase #2 $158.74   13.719%   $180.5175   $180.52   $180.50   $180.50  
  Purchase #3 $209.47   11.126%   $232.7756   $232.78   $232.80   $232.80  
  Purchase #4 $61.31   13.138%   $69.3649   $69.36   $69.35   $69.40  
  Purchase #5 $56.99   6.267%   $60.5616   $60.56   $60.55   $60.60  
  Purchase #6 $91.67   10.241%   $101.0579   $101.06   $101.05   $101.10  
  Purchase #7 $109.80   5.172%   $115.4789   $115.48   $115.50   $115.50  
  7 purchase totals = $924.43       $1,026.0298   $1,026.03   $1,026.00   $1,026.20