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 $9.65   8.499%   $10.4702   $10.47   $10.45   $10.50  
  Purchase #2 $17.25   7.231%   $18.4973   $18.50   $18.50   $18.50  
  Purchase #3 $137.48   12.849%   $155.1448   $155.14   $155.15   $155.10  
  Purchase #4 $112.81   13.567%   $128.1149   $128.11   $128.10   $128.10  
  Purchase #5 $85.83   10.491%   $94.8344   $94.83   $94.85   $94.80  
  Purchase #6 $199.26   10.295%   $219.7738   $219.77   $219.75   $219.80  
  Purchase #7 $9.47   10.529%   $10.4671   $10.47   $10.45   $10.50  
  7 purchase totals = $571.75       $637.3025   $637.29   $637.25   $637.30