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 $25.57   12.275%   $28.7087   $28.71   $28.70   $28.70  
  Purchase #2 $149.27   12.762%   $168.3198   $168.32   $168.30   $168.30  
  Purchase #3 $158.85   8.783%   $172.8018   $172.80   $172.80   $172.80  
  Purchase #4 $41.55   13.572%   $47.1892   $47.19   $47.20   $47.20  
  Purchase #5 $161.42   8.376%   $174.9405   $174.94   $174.95   $174.90  
  Purchase #6 $6.98   12.396%   $7.8452   $7.85   $7.85   $7.80  
  Purchase #7 $135.75   3.206%   $140.1021   $140.10   $140.10   $140.10  
  7 purchase totals = $679.39       $739.9073   $739.91   $739.90   $739.80