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 $15.91   8.261%   $17.2243   $17.22   $17.20   $17.20  
  Purchase #2 $111.71   8.279%   $120.9585   $120.96   $120.95   $121.00  
  Purchase #3 $101.83   3.459%   $105.3523   $105.35   $105.35   $105.40  
  Purchase #4 $241.97   12.183%   $271.4492   $271.45   $271.45   $271.40  
  Purchase #5 $110.83   7.470%   $119.109   $119.11   $119.10   $119.10  
  Purchase #6 $212.72   12.676%   $239.6844   $239.68   $239.70   $239.70  
  Purchase #7 $125.41   8.239%   $135.7425   $135.74   $135.75   $135.70  
  7 purchase totals = $920.38       $1,009.5202   $1,009.51   $1,009.50   $1,009.50