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 $240.75   12.612%   $271.1134   $271.11   $271.10   $271.10  
  Purchase #2 $149.40   6.356%   $158.8959   $158.90   $158.90   $158.90  
  Purchase #3 $68.57   11.215%   $76.2601   $76.26   $76.25   $76.30  
  Purchase #4 $149.74   12.493%   $168.447   $168.45   $168.45   $168.40  
  Purchase #5 $84.52   8.394%   $91.6146   $91.61   $91.60   $91.60  
  Purchase #6 $37.70   9.118%   $41.1375   $41.14   $41.15   $41.10  
  Purchase #7 $175.62   10.408%   $193.8985   $193.90   $193.90   $193.90  
  7 purchase totals = $906.30       $1,001.3670   $1,001.37   $1,001.35   $1,001.30