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 $78.96   12.502%   $88.8316   $88.83   $88.85   $88.80  
  Purchase #2 $98.92   8.479%   $107.3074   $107.31   $107.30   $107.30  
  Purchase #3 $208.95   6.331%   $222.1786   $222.18   $222.20   $222.20  
  Purchase #4 $134.79   4.191%   $140.439   $140.44   $140.45   $140.40  
  Purchase #5 $54.57   11.631%   $60.917   $60.92   $60.90   $60.90  
  Purchase #6 $128.71   12.393%   $144.661   $144.66   $144.65   $144.70  
  Purchase #7 $126.81   9.621%   $139.0104   $139.01   $139.00   $139.00  
  7 purchase totals = $831.71       $903.3450   $903.35   $903.35   $903.30