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 $98.77   8.428%   $107.0943   $107.09   $107.10   $107.10  
  Purchase #2 $77.28   3.471%   $79.9624   $79.96   $79.95   $80.00  
  Purchase #3 $123.46   7.868%   $133.1738   $133.17   $133.15   $133.20  
  Purchase #4 $15.25   3.508%   $15.785   $15.79   $15.80   $15.80  
  Purchase #5 $36.71   3.594%   $38.0294   $38.03   $38.05   $38.00  
  Purchase #6 $39.96   11.160%   $44.4195   $44.42   $44.40   $44.40  
  Purchase #7 $209.96   9.610%   $230.1372   $230.14   $230.15   $230.10  
  7 purchase totals = $601.39       $648.6016   $648.60   $648.60   $648.60