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 $245.61   13.718%   $279.3028   $279.30   $279.30   $279.30  
  Purchase #2 $244.98   11.288%   $272.6333   $272.63   $272.65   $272.60  
  Purchase #3 $233.55   3.353%   $241.3809   $241.38   $241.40   $241.40  
  Purchase #4 $236.51   13.554%   $268.5666   $268.57   $268.55   $268.60  
  Purchase #5 $68.77   12.680%   $77.49   $77.49   $77.50   $77.50  
  Purchase #6 $11.65   13.244%   $13.1929   $13.19   $13.20   $13.20  
  Purchase #7 $205.62   10.685%   $227.5905   $227.59   $227.60   $227.60  
  7 purchase totals = $1,246.69       $1,380.1570   $1,380.15   $1,380.20   $1,380.20