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 $250.67   8.321%   $271.5283   $271.53   $271.55   $271.50  
  Purchase #2 $81.96   12.283%   $92.0271   $92.03   $92.05   $92.00  
  Purchase #3 $178.34   5.645%   $188.4073   $188.41   $188.40   $188.40  
  Purchase #4 $38.28   6.284%   $40.6855   $40.69   $40.70   $40.70  
  Purchase #5 $145.25   6.365%   $154.4952   $154.50   $154.50   $154.50  
  Purchase #6 $246.55   8.823%   $268.3031   $268.30   $268.30   $268.30  
  Purchase #7 $223.71   11.122%   $248.591   $248.59   $248.60   $248.60  
  7 purchase totals = $1,164.76       $1,264.0375   $1,264.05   $1,264.10   $1,264.00