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 $18.88   10.856%   $20.9296   $20.93   $20.95   $20.90  
  Purchase #2 $80.74   13.229%   $91.4211   $91.42   $91.40   $91.40  
  Purchase #3 $140.61   11.141%   $156.2754   $156.28   $156.30   $156.30  
  Purchase #4 $211.99   6.720%   $226.2357   $226.24   $226.25   $226.20  
  Purchase #5 $243.65   10.554%   $269.3648   $269.36   $269.35   $269.40  
  Purchase #6 $247.89   9.496%   $271.4296   $271.43   $271.45   $271.40  
  Purchase #7 $228.68   4.701%   $239.4302   $239.43   $239.45   $239.40  
  7 purchase totals = $1,172.44       $1,275.0864   $1,275.09   $1,275.15   $1,275.00