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 $20.71   13.634%   $23.5336   $23.53   $23.55   $23.50  
  Purchase #2 $100.78   11.757%   $112.6287   $112.63   $112.65   $112.60  
  Purchase #3 $222.29   11.580%   $248.0312   $248.03   $248.05   $248.00  
  Purchase #4 $23.30   4.764%   $24.41   $24.41   $24.40   $24.40  
  Purchase #5 $162.60   6.671%   $173.447   $173.45   $173.45   $173.40  
  Purchase #6 $65.87   5.284%   $69.3506   $69.35   $69.35   $69.40  
  Purchase #7 $168.63   9.387%   $184.4593   $184.46   $184.45   $184.50  
  7 purchase totals = $764.18       $835.8604   $835.86   $835.90   $835.80