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 $38.25   10.412%   $42.2326   $42.23   $42.25   $42.20  
  Purchase #2 $74.85   7.193%   $80.234   $80.23   $80.25   $80.20  
  Purchase #3 $16.64   10.490%   $18.3855   $18.39   $18.40   $18.40  
  Purchase #4 $37.64   10.819%   $41.7123   $41.71   $41.70   $41.70  
  Purchase #5 $158.83   6.656%   $169.4017   $169.40   $169.40   $169.40  
  Purchase #6 $71.90   13.442%   $81.5648   $81.56   $81.55   $81.60  
  Purchase #7 $191.29   8.250%   $207.0714   $207.07   $207.05   $207.10  
  7 purchase totals = $589.40       $640.6023   $640.59   $640.60   $640.60