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 $7.46   9.858%   $8.1954   $8.20   $8.20   $8.20  
  Purchase #2 $34.82   6.474%   $37.0742   $37.07   $37.05   $37.10  
  Purchase #3 $239.68   3.478%   $248.0161   $248.02   $248.00   $248.00  
  Purchase #4 $16.70   10.305%   $18.4209   $18.42   $18.40   $18.40  
  Purchase #5 $140.41   11.585%   $156.6765   $156.68   $156.70   $156.70  
  Purchase #6 $175.94   12.590%   $198.0908   $198.09   $198.10   $198.10  
  Purchase #7 $233.71   8.648%   $253.9212   $253.92   $253.90   $253.90  
  7 purchase totals = $848.72       $920.3951   $920.40   $920.35   $920.40