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 $18.71   6.796%   $19.9815   $19.98   $20.00   $20.00  
  Purchase #2 $5.47   12.807%   $6.1705   $6.17   $6.15   $6.20  
  Purchase #3 $116.85   6.559%   $124.5142   $124.51   $124.50   $124.50  
  Purchase #4 $115.80   9.446%   $126.7385   $126.74   $126.75   $126.70  
  Purchase #5 $35.76   4.135%   $37.2387   $37.24   $37.25   $37.20  
  Purchase #6 $117.36   8.559%   $127.4048   $127.40   $127.40   $127.40  
  Purchase #7 $35.56   3.421%   $36.7765   $36.78   $36.80   $36.80  
  7 purchase totals = $445.51       $478.8247   $478.82   $478.85   $478.80