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 $185.53   12.207%   $208.1776   $208.18   $208.20   $208.20  
  Purchase #2 $14.62   5.572%   $15.4346   $15.43   $15.45   $15.40  
  Purchase #3 $34.53   6.272%   $36.6957   $36.70   $36.70   $36.70  
  Purchase #4 $163.93   12.764%   $184.854   $184.85   $184.85   $184.90  
  Purchase #5 $114.40   11.824%   $127.9267   $127.93   $127.95   $127.90  
  Purchase #6 $96.70   7.560%   $104.0105   $104.01   $104.00   $104.00  
  Purchase #7 $149.56   12.198%   $167.8033   $167.80   $167.80   $167.80  
  7 purchase totals = $759.27       $844.9024   $844.90   $844.95   $844.90