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 $30.62   3.813%   $31.7875   $31.79   $31.80   $31.80  
  Purchase #2 $117.60   13.436%   $133.4007   $133.40   $133.40   $133.40  
  Purchase #3 $18.54   13.380%   $21.0207   $21.02   $21.00   $21.00  
  Purchase #4 $198.56   9.332%   $217.0896   $217.09   $217.10   $217.10  
  Purchase #5 $24.40   13.525%   $27.7001   $27.70   $27.70   $27.70  
  Purchase #6 $46.70   6.780%   $49.8663   $49.87   $49.85   $49.90  
  Purchase #7 $64.74   7.406%   $69.5346   $69.53   $69.55   $69.50  
  7 purchase totals = $501.16       $550.3995   $550.40   $550.40   $550.40