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 $56.34   3.544%   $58.3367   $58.34   $58.35   $58.30  
  Purchase #2 $119.28   8.122%   $128.9679   $128.97   $128.95   $129.00  
  Purchase #3 $208.61   8.365%   $226.0602   $226.06   $226.05   $226.10  
  Purchase #4 $96.41   6.490%   $102.667   $102.67   $102.65   $102.70  
  Purchase #5 $9.57   9.159%   $10.4465   $10.45   $10.45   $10.40  
  Purchase #6 $59.89   7.469%   $64.3632   $64.36   $64.35   $64.40  
  Purchase #7 $11.61   5.278%   $12.2228   $12.22   $12.20   $12.20  
  7 purchase totals = $561.71       $603.0643   $603.07   $603.00   $603.10