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 $74.95   12.753%   $84.5084   $84.51   $84.50   $84.50  
  Purchase #2 $53.89   10.206%   $59.39   $59.39   $59.40   $59.40  
  Purchase #3 $201.35   6.591%   $214.621   $214.62   $214.60   $214.60  
  Purchase #4 $56.79   11.147%   $63.1204   $63.12   $63.10   $63.10  
  Purchase #5 $171.99   3.357%   $177.7637   $177.76   $177.75   $177.80  
  Purchase #6 $107.46   11.234%   $119.5321   $119.53   $119.55   $119.50  
  Purchase #7 $225.90   6.261%   $240.0436   $240.04   $240.05   $240.00  
  7 purchase totals = $892.33       $958.9792   $958.97   $958.95   $958.90