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 $114.67   6.105%   $121.6706   $121.67   $121.65   $121.70  
  Purchase #2 $151.77   12.149%   $170.2085   $170.21   $170.20   $170.20  
  Purchase #3 $246.28   12.346%   $276.6857   $276.69   $276.70   $276.70  
  Purchase #4 $136.63   7.744%   $147.2106   $147.21   $147.20   $147.20  
  Purchase #5 $72.29   13.806%   $82.2704   $82.27   $82.25   $82.30  
  Purchase #6 $60.88   11.310%   $67.7655   $67.77   $67.75   $67.80  
  Purchase #7 $49.84   6.367%   $53.0133   $53.01   $53.00   $53.00  
  7 purchase totals = $832.36       $918.8246   $918.83   $918.75   $918.90