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 $248.50   6.199%   $263.9045   $263.90   $263.90   $263.90  
  Purchase #2 $66.29   13.141%   $75.0012   $75.00   $75.00   $75.00  
  Purchase #3 $23.29   12.469%   $26.194   $26.19   $26.20   $26.20  
  Purchase #4 $114.65   7.679%   $123.454   $123.45   $123.45   $123.50  
  Purchase #5 $118.41   4.497%   $123.7349   $123.73   $123.75   $123.70  
  Purchase #6 $174.86   10.574%   $193.3497   $193.35   $193.35   $193.30  
  Purchase #7 $28.39   6.782%   $30.3154   $30.32   $30.30   $30.30  
  7 purchase totals = $774.39       $835.9537   $835.94   $835.95   $835.90