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 $55.93   13.191%   $63.3077   $63.31   $63.30   $63.30  
  Purchase #2 $142.58   4.527%   $149.0346   $149.03   $149.05   $149.00  
  Purchase #3 $170.89   4.577%   $178.7116   $178.71   $178.70   $178.70  
  Purchase #4 $157.86   11.358%   $175.7897   $175.79   $175.80   $175.80  
  Purchase #5 $207.52   13.212%   $234.9375   $234.94   $234.95   $234.90  
  Purchase #6 $38.47   4.318%   $40.1311   $40.13   $40.15   $40.10  
  Purchase #7 $166.43   12.618%   $187.4301   $187.43   $187.45   $187.40  
  7 purchase totals = $939.68       $1,029.3423   $1,029.34   $1,029.40   $1,029.20