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 $240.82   10.675%   $266.5275   $266.53   $266.55   $266.50  
  Purchase #2 $87.52   7.332%   $93.937   $93.94   $93.95   $93.90  
  Purchase #3 $16.90   7.292%   $18.1323   $18.13   $18.15   $18.10  
  Purchase #4 $250.77   9.413%   $274.375   $274.38   $274.40   $274.40  
  Purchase #5 $151.43   6.366%   $161.07   $161.07   $161.05   $161.10  
  Purchase #6 $93.67   3.811%   $97.2398   $97.24   $97.25   $97.20  
  Purchase #7 $60.38   12.640%   $68.012   $68.01   $68.00   $68.00  
  7 purchase totals = $901.49       $979.2936   $979.30   $979.35   $979.20