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 $203.43   7.274%   $218.2275   $218.23   $218.25   $218.20  
  Purchase #2 $6.97   5.620%   $7.3617   $7.36   $7.35   $7.40  
  Purchase #3 $26.92   6.419%   $28.648   $28.65   $28.65   $28.60  
  Purchase #4 $201.34   3.773%   $208.9366   $208.94   $208.95   $208.90  
  Purchase #5 $207.37   10.309%   $228.7478   $228.75   $228.75   $228.70  
  Purchase #6 $200.50   12.695%   $225.9535   $225.95   $225.95   $226.00  
  Purchase #7 $8.68   12.603%   $9.7739   $9.77   $9.75   $9.80  
  7 purchase totals = $855.21       $927.6490   $927.65   $927.65   $927.60