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 $18.36   3.417%   $18.9874   $18.99   $19.00   $19.00  
  Purchase #2 $95.43   12.800%   $107.645   $107.65   $107.65   $107.60  
  Purchase #3 $194.60   12.441%   $218.8102   $218.81   $218.80   $218.80  
  Purchase #4 $37.66   5.269%   $39.6443   $39.64   $39.65   $39.60  
  Purchase #5 $24.59   12.787%   $27.7343   $27.73   $27.75   $27.70  
  Purchase #6 $71.84   10.794%   $79.5944   $79.59   $79.60   $79.60  
  Purchase #7 $56.58   6.301%   $60.1451   $60.15   $60.15   $60.10  
  7 purchase totals = $499.06       $552.5607   $552.56   $552.60   $552.40