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 $243.89   13.241%   $276.1835   $276.18   $276.20   $276.20  
  Purchase #2 $226.91   7.213%   $243.277   $243.28   $243.30   $243.30  
  Purchase #3 $73.57   5.375%   $77.5244   $77.52   $77.50   $77.50  
  Purchase #4 $233.74   3.175%   $241.1612   $241.16   $241.15   $241.20  
  Purchase #5 $154.64   5.712%   $163.473   $163.47   $163.45   $163.50  
  Purchase #6 $65.85   7.168%   $70.5701   $70.57   $70.55   $70.60  
  Purchase #7 $208.49   13.529%   $236.6966   $236.70   $236.70   $236.70  
  7 purchase totals = $1,207.09       $1,308.8858   $1,308.88   $1,308.85   $1,309.00