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 $133.31   4.318%   $139.0663   $139.07   $139.05   $139.10  
  Purchase #2 $60.93   10.103%   $67.0858   $67.09   $67.10   $67.10  
  Purchase #3 $36.28   7.787%   $39.1051   $39.11   $39.10   $39.10  
  Purchase #4 $71.36   6.701%   $76.1418   $76.14   $76.15   $76.10  
  Purchase #5 $168.69   13.354%   $191.2169   $191.22   $191.20   $191.20  
  Purchase #6 $49.86   5.831%   $52.7673   $52.77   $52.75   $52.80  
  Purchase #7 $160.30   10.457%   $177.0626   $177.06   $177.05   $177.10  
  7 purchase totals = $680.73       $742.4458   $742.46   $742.40   $742.50