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 $70.45   11.792%   $78.7575   $78.76   $78.75   $78.80  
  Purchase #2 $248.54   13.118%   $281.1435   $281.14   $281.15   $281.10  
  Purchase #3 $224.27   8.657%   $243.6851   $243.69   $243.70   $243.70  
  Purchase #4 $204.67   4.796%   $214.486   $214.49   $214.50   $214.50  
  Purchase #5 $145.74   8.344%   $157.9005   $157.90   $157.90   $157.90  
  Purchase #6 $222.26   9.731%   $243.8881   $243.89   $243.90   $243.90  
  Purchase #7 $23.94   9.670%   $26.255   $26.26   $26.25   $26.30  
  7 purchase totals = $1,139.87       $1,246.1157   $1,246.13   $1,246.15   $1,246.20