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 $17.30   6.594%   $18.4408   $18.44   $18.45   $18.40  
  Purchase #2 $74.75   11.405%   $83.2752   $83.28   $83.30   $83.30  
  Purchase #3 $164.81   12.483%   $185.3832   $185.38   $185.40   $185.40  
  Purchase #4 $191.60   11.765%   $214.1417   $214.14   $214.15   $214.10  
  Purchase #5 $194.92   3.195%   $201.1477   $201.15   $201.15   $201.10  
  Purchase #6 $202.39   4.175%   $210.8398   $210.84   $210.85   $210.80  
  Purchase #7 $57.48   3.698%   $59.6056   $59.61   $59.60   $59.60  
  7 purchase totals = $903.25       $972.8340   $972.84   $972.90   $972.70