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 $21.63   13.347%   $24.517   $24.52   $24.50   $24.50  
  Purchase #2 $104.68   11.665%   $116.8909   $116.89   $116.90   $116.90  
  Purchase #3 $89.60   9.177%   $97.8226   $97.82   $97.80   $97.80  
  Purchase #4 $53.51   13.184%   $60.5648   $60.56   $60.55   $60.60  
  Purchase #5 $172.42   5.362%   $181.6652   $181.67   $181.65   $181.70  
  Purchase #6 $233.50   3.287%   $241.1751   $241.18   $241.20   $241.20  
  Purchase #7 $246.49   5.385%   $259.7635   $259.76   $259.75   $259.80  
  7 purchase totals = $921.83       $982.3991   $982.40   $982.35   $982.50