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 $67.34   11.853%   $75.3218   $75.32   $75.30   $75.30  
  Purchase #2 $52.63   4.385%   $54.9378   $54.94   $54.95   $54.90  
  Purchase #3 $111.78   6.657%   $119.2212   $119.22   $119.20   $119.20  
  Purchase #4 $80.93   7.679%   $87.1446   $87.14   $87.15   $87.10  
  Purchase #5 $205.69   12.178%   $230.7389   $230.74   $230.75   $230.70  
  Purchase #6 $53.86   12.285%   $60.4767   $60.48   $60.50   $60.50  
  Purchase #7 $26.77   13.476%   $30.3775   $30.38   $30.40   $30.40  
  7 purchase totals = $599.00       $658.2185   $658.22   $658.25   $658.10