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 $91.36   10.449%   $100.9062   $100.91   $100.90   $100.90  
  Purchase #2 $144.34   12.280%   $162.065   $162.07   $162.05   $162.10  
  Purchase #3 $238.36   5.387%   $251.2005   $251.20   $251.20   $251.20  
  Purchase #4 $136.99   11.248%   $152.3986   $152.40   $152.40   $152.40  
  Purchase #5 $107.65   9.643%   $118.0307   $118.03   $118.05   $118.00  
  Purchase #6 $176.38   6.493%   $187.8324   $187.83   $187.85   $187.80  
  Purchase #7 $24.69   11.445%   $27.5158   $27.52   $27.50   $27.50  
  7 purchase totals = $919.77       $999.9492   $999.96   $999.95   $999.90