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 $247.75   10.449%   $273.6374   $273.64   $273.65   $273.60  
  Purchase #2 $248.30   5.804%   $262.7113   $262.71   $262.70   $262.70  
  Purchase #3 $235.85   4.657%   $246.8335   $246.83   $246.85   $246.80  
  Purchase #4 $77.56   7.239%   $83.1746   $83.17   $83.15   $83.20  
  Purchase #5 $96.89   11.751%   $108.2755   $108.28   $108.30   $108.30  
  Purchase #6 $113.57   9.808%   $124.7089   $124.71   $124.70   $124.70  
  Purchase #7 $190.98   6.604%   $203.5923   $203.59   $203.60   $203.60  
  7 purchase totals = $1,210.90       $1,302.9335   $1,302.93   $1,302.95   $1,302.90