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 $250.38   6.327%   $266.2215   $266.22   $266.20   $266.20  
  Purchase #2 $207.98   9.728%   $228.2123   $228.21   $228.20   $228.20  
  Purchase #3 $236.57   6.803%   $252.6639   $252.66   $252.65   $252.70  
  Purchase #4 $249.62   6.402%   $265.6007   $265.60   $265.60   $265.60  
  Purchase #5 $168.33   13.637%   $191.2852   $191.29   $191.30   $191.30  
  Purchase #6 $48.79   9.706%   $53.5256   $53.53   $53.55   $53.50  
  Purchase #7 $114.36   5.554%   $120.7116   $120.71   $120.70   $120.70  
  7 purchase totals = $1,276.03       $1,378.2208   $1,378.22   $1,378.20   $1,378.20