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 $8.33   3.664%   $8.6352   $8.64   $8.65   $8.60  
  Purchase #2 $216.64   7.600%   $233.1046   $233.10   $233.10   $233.10  
  Purchase #3 $182.50   8.592%   $198.1804   $198.18   $198.20   $198.20  
  Purchase #4 $149.38   5.517%   $157.6213   $157.62   $157.60   $157.60  
  Purchase #5 $108.79   11.603%   $121.4129   $121.41   $121.40   $121.40  
  Purchase #6 $139.82   13.743%   $159.0355   $159.04   $159.05   $159.00  
  Purchase #7 $104.82   7.400%   $112.5767   $112.58   $112.60   $112.60  
  7 purchase totals = $910.28       $990.5666   $990.57   $990.60   $990.50