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 $23.90   12.332%   $26.8473   $26.85   $26.85   $26.80  
  Purchase #2 $71.98   11.549%   $80.293   $80.29   $80.30   $80.30  
  Purchase #3 $116.77   3.463%   $120.8137   $120.81   $120.80   $120.80  
  Purchase #4 $54.39   8.806%   $59.1796   $59.18   $59.20   $59.20  
  Purchase #5 $226.77   4.221%   $236.342   $236.34   $236.35   $236.30  
  Purchase #6 $244.40   7.865%   $263.6221   $263.62   $263.60   $263.60  
  Purchase #7 $129.83   3.774%   $134.7298   $134.73   $134.75   $134.70  
  7 purchase totals = $868.04       $921.8275   $921.82   $921.85   $921.70