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 $241.48   12.352%   $271.3076   $271.31   $271.30   $271.30  
  Purchase #2 $146.30   7.569%   $157.3734   $157.37   $157.35   $157.40  
  Purchase #3 $173.57   12.359%   $195.0215   $195.02   $195.00   $195.00  
  Purchase #4 $226.70   9.645%   $248.5652   $248.57   $248.55   $248.60  
  Purchase #5 $244.54   3.608%   $253.363   $253.36   $253.35   $253.40  
  Purchase #6 $78.25   8.387%   $84.8128   $84.81   $84.80   $84.80  
  Purchase #7 $246.53   7.282%   $264.4823   $264.48   $264.50   $264.50  
  7 purchase totals = $1,357.37       $1,474.9258   $1,474.92   $1,474.85   $1,475.00