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 $246.69   13.398%   $279.7415   $279.74   $279.75   $279.70  
  Purchase #2 $96.27   6.392%   $102.4236   $102.42   $102.40   $102.40  
  Purchase #3 $225.96   4.557%   $236.257   $236.26   $236.25   $236.30  
  Purchase #4 $213.76   5.653%   $225.8439   $225.84   $225.85   $225.80  
  Purchase #5 $164.60   9.466%   $180.181   $180.18   $180.20   $180.20  
  Purchase #6 $199.61   10.158%   $219.8864   $219.89   $219.90   $219.90  
  Purchase #7 $32.67   8.103%   $35.3173   $35.32   $35.30   $35.30  
  7 purchase totals = $1,179.56       $1,279.6507   $1,279.65   $1,279.65   $1,279.60