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 $27.88   10.813%   $30.8947   $30.89   $30.90   $30.90  
  Purchase #2 $73.31   5.168%   $77.0987   $77.10   $77.10   $77.10  
  Purchase #3 $30.38   9.795%   $33.3557   $33.36   $33.35   $33.40  
  Purchase #4 $136.86   7.463%   $147.0739   $147.07   $147.05   $147.10  
  Purchase #5 $146.48   13.692%   $166.536   $166.54   $166.55   $166.50  
  Purchase #6 $131.78   3.695%   $136.6493   $136.65   $136.65   $136.60  
  Purchase #7 $17.39   9.406%   $19.0257   $19.03   $19.05   $19.00  
  7 purchase totals = $564.08       $610.6340   $610.64   $610.65   $610.60