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 $37.98   11.858%   $42.4837   $42.48   $42.50   $42.50  
  Purchase #2 $22.71   7.348%   $24.3787   $24.38   $24.40   $24.40  
  Purchase #3 $194.82   12.189%   $218.5666   $218.57   $218.55   $218.60  
  Purchase #4 $192.59   11.633%   $214.994   $214.99   $215.00   $215.00  
  Purchase #5 $88.60   8.363%   $96.0096   $96.01   $96.00   $96.00  
  Purchase #6 $241.69   8.632%   $262.5527   $262.55   $262.55   $262.60  
  Purchase #7 $147.53   3.219%   $152.279   $152.28   $152.30   $152.30  
  7 purchase totals = $925.92       $1,011.2643   $1,011.26   $1,011.30   $1,011.40