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 $243.51   13.219%   $275.6996   $275.70   $275.70   $275.70  
  Purchase #2 $114.41   4.326%   $119.3594   $119.36   $119.35   $119.40  
  Purchase #3 $29.72   10.706%   $32.9018   $32.90   $32.90   $32.90  
  Purchase #4 $97.90   12.320%   $109.9613   $109.96   $109.95   $110.00  
  Purchase #5 $119.44   11.392%   $133.0466   $133.05   $133.05   $133.00  
  Purchase #6 $132.45   11.767%   $148.0354   $148.04   $148.05   $148.00  
  Purchase #7 $20.38   6.514%   $21.7076   $21.71   $21.70   $21.70  
  7 purchase totals = $757.81       $840.7117   $840.72   $840.70   $840.70