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 $226.46   10.412%   $250.039   $250.04   $250.05   $250.00  
  Purchase #2 $91.80   7.416%   $98.6079   $98.61   $98.60   $98.60  
  Purchase #3 $91.57   13.271%   $103.7223   $103.72   $103.70   $103.70  
  Purchase #4 $115.53   9.864%   $126.9259   $126.93   $126.95   $126.90  
  Purchase #5 $126.41   3.349%   $130.6435   $130.64   $130.65   $130.60  
  Purchase #6 $157.30   12.605%   $177.1277   $177.13   $177.15   $177.10  
  Purchase #7 $105.67   4.568%   $110.497   $110.50   $110.50   $110.50  
  7 purchase totals = $914.74       $997.5633   $997.57   $997.60   $997.40