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 $18.83   6.181%   $19.9939   $19.99   $20.00   $20.00  
  Purchase #2 $155.33   3.673%   $161.0353   $161.04   $161.05   $161.00  
  Purchase #3 $36.88   12.111%   $41.3465   $41.35   $41.35   $41.30  
  Purchase #4 $164.58   12.307%   $184.8349   $184.83   $184.85   $184.80  
  Purchase #5 $106.35   11.407%   $118.4813   $118.48   $118.50   $118.50  
  Purchase #6 $159.25   9.450%   $174.2991   $174.30   $174.30   $174.30  
  Purchase #7 $249.91   11.635%   $278.987   $278.99   $279.00   $279.00  
  7 purchase totals = $891.13       $978.9780   $978.98   $979.05   $978.90