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 $120.94   7.784%   $130.354   $130.35   $130.35   $130.40  
  Purchase #2 $26.83   12.492%   $30.1816   $30.18   $30.20   $30.20  
  Purchase #3 $235.76   8.359%   $255.4672   $255.47   $255.45   $255.50  
  Purchase #4 $26.28   8.168%   $28.4266   $28.43   $28.45   $28.40  
  Purchase #5 $132.84   4.411%   $138.6996   $138.70   $138.70   $138.70  
  Purchase #6 $74.72   4.694%   $78.2274   $78.23   $78.25   $78.20  
  Purchase #7 $7.77   13.773%   $8.8402   $8.84   $8.85   $8.80  
  7 purchase totals = $625.14       $670.1966   $670.20   $670.25   $670.20