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 $12.40   6.159%   $13.1637   $13.16   $13.15   $13.20  
  Purchase #2 $8.98   9.706%   $9.8516   $9.85   $9.85   $9.90  
  Purchase #3 $97.69   8.510%   $106.0034   $106.00   $106.00   $106.00  
  Purchase #4 $220.57   9.810%   $242.2079   $242.21   $242.20   $242.20  
  Purchase #5 $171.59   10.205%   $189.1008   $189.10   $189.10   $189.10  
  Purchase #6 $53.63   13.739%   $60.9982   $61.00   $61.00   $61.00  
  Purchase #7 $221.94   11.353%   $247.1368   $247.14   $247.15   $247.10  
  7 purchase totals = $786.80       $868.4624   $868.46   $868.45   $868.50