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 $30.35   4.701%   $31.7768   $31.78   $31.80   $31.80  
  Purchase #2 $40.25   10.345%   $44.4139   $44.41   $44.40   $44.40  
  Purchase #3 $67.54   5.817%   $71.4688   $71.47   $71.45   $71.50  
  Purchase #4 $36.54   5.232%   $38.4518   $38.45   $38.45   $38.50  
  Purchase #5 $106.29   7.648%   $114.4191   $114.42   $114.40   $114.40  
  Purchase #6 $158.62   11.733%   $177.2309   $177.23   $177.25   $177.20  
  Purchase #7 $109.67   11.204%   $121.9574   $121.96   $121.95   $122.00  
  7 purchase totals = $549.26       $599.7187   $599.72   $599.70   $599.80