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 $226.75   10.234%   $249.9556   $249.96   $249.95   $250.00  
  Purchase #2 $164.59   5.568%   $173.7544   $173.75   $173.75   $173.80  
  Purchase #3 $73.62   8.358%   $79.7732   $79.77   $79.75   $79.80  
  Purchase #4 $236.33   6.610%   $251.9514   $251.95   $251.95   $252.00  
  Purchase #5 $201.31   12.344%   $226.1597   $226.16   $226.15   $226.20  
  Purchase #6 $157.27   13.854%   $179.0582   $179.06   $179.05   $179.10  
  Purchase #7 $75.89   6.287%   $80.6612   $80.66   $80.65   $80.70  
  7 purchase totals = $1,135.76       $1,241.3137   $1,241.31   $1,241.25   $1,241.60