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 $16.76   9.851%   $18.411   $18.41   $18.40   $18.40  
  Purchase #2 $20.74   11.122%   $23.0467   $23.05   $23.05   $23.00  
  Purchase #3 $181.35   12.405%   $203.8465   $203.85   $203.85   $203.80  
  Purchase #4 $16.56   4.358%   $17.2817   $17.28   $17.30   $17.30  
  Purchase #5 $70.97   8.312%   $76.869   $76.87   $76.85   $76.90  
  Purchase #6 $121.90   7.162%   $130.6305   $130.63   $130.65   $130.60  
  Purchase #7 $95.25   13.245%   $107.8659   $107.87   $107.85   $107.90  
  7 purchase totals = $523.53       $577.9513   $577.96   $577.95   $577.90