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 $113.51   13.156%   $128.4434   $128.44   $128.45   $128.40  
  Purchase #2 $130.90   6.838%   $139.8509   $139.85   $139.85   $139.90  
  Purchase #3 $233.36   10.700%   $258.3295   $258.33   $258.35   $258.30  
  Purchase #4 $28.97   5.647%   $30.6059   $30.61   $30.60   $30.60  
  Purchase #5 $121.31   11.587%   $135.3662   $135.37   $135.35   $135.40  
  Purchase #6 $227.49   7.508%   $244.5699   $244.57   $244.55   $244.60  
  Purchase #7 $47.93   12.110%   $53.7343   $53.73   $53.75   $53.70  
  7 purchase totals = $903.47       $990.9001   $990.90   $990.90   $990.90