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 $79.66   11.181%   $88.5668   $88.57   $88.55   $88.60  
  Purchase #2 $5.89   6.255%   $6.2584   $6.26   $6.25   $6.30  
  Purchase #3 $43.57   13.865%   $49.611   $49.61   $49.60   $49.60  
  Purchase #4 $153.73   8.833%   $167.309   $167.31   $167.30   $167.30  
  Purchase #5 $74.96   5.459%   $79.0521   $79.05   $79.05   $79.10  
  Purchase #6 $102.96   10.159%   $113.4197   $113.42   $113.40   $113.40  
  Purchase #7 $231.57   6.130%   $245.7652   $245.77   $245.75   $245.80  
  7 purchase totals = $692.34       $749.9822   $749.99   $749.90   $750.10