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 $40.99   3.615%   $42.4718   $42.47   $42.45   $42.50  
  Purchase #2 $18.57   7.603%   $19.9819   $19.98   $20.00   $20.00  
  Purchase #3 $215.58   6.519%   $229.6337   $229.63   $229.65   $229.60  
  Purchase #4 $232.88   8.285%   $252.1741   $252.17   $252.15   $252.20  
  Purchase #5 $200.33   3.385%   $207.1112   $207.11   $207.10   $207.10  
  Purchase #6 $103.98   13.408%   $117.9216   $117.92   $117.90   $117.90  
  Purchase #7 $49.85   9.678%   $54.6745   $54.67   $54.65   $54.70  
  7 purchase totals = $862.18       $923.9688   $923.95   $923.90   $924.00