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 $248.32   12.575%   $279.5462   $279.55   $279.55   $279.50  
  Purchase #2 $190.83   4.192%   $198.8296   $198.83   $198.85   $198.80  
  Purchase #3 $31.25   13.295%   $35.4047   $35.40   $35.40   $35.40  
  Purchase #4 $99.32   5.237%   $104.5214   $104.52   $104.50   $104.50  
  Purchase #5 $208.93   7.722%   $225.0636   $225.06   $225.05   $225.10  
  Purchase #6 $117.93   6.261%   $125.3136   $125.31   $125.30   $125.30  
  Purchase #7 $222.43   4.633%   $232.7352   $232.74   $232.75   $232.70  
  7 purchase totals = $1,119.01       $1,201.4143   $1,201.41   $1,201.40   $1,201.30