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 $248.51   9.181%   $271.3257   $271.33   $271.35   $271.30  
  Purchase #2 $55.38   4.770%   $58.0216   $58.02   $58.00   $58.00  
  Purchase #3 $50.90   4.685%   $53.2847   $53.28   $53.30   $53.30  
  Purchase #4 $91.31   6.369%   $97.1255   $97.13   $97.15   $97.10  
  Purchase #5 $243.71   6.268%   $258.9857   $258.99   $259.00   $259.00  
  Purchase #6 $37.47   8.474%   $40.6452   $40.65   $40.65   $40.60  
  Purchase #7 $99.46   9.518%   $108.9266   $108.93   $108.95   $108.90  
  7 purchase totals = $826.74       $888.3150   $888.33   $888.40   $888.20