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 $104.49   12.714%   $117.7749   $117.77   $117.75   $117.80  
  Purchase #2 $55.92   8.375%   $60.6033   $60.60   $60.60   $60.60  
  Purchase #3 $166.27   7.486%   $178.717   $178.72   $178.70   $178.70  
  Purchase #4 $60.71   5.195%   $63.8639   $63.86   $63.85   $63.90  
  Purchase #5 $5.74   11.387%   $6.3936   $6.39   $6.40   $6.40  
  Purchase #6 $61.71   11.855%   $69.0257   $69.03   $69.05   $69.00  
  Purchase #7 $110.34   7.547%   $118.6674   $118.67   $118.65   $118.70  
  7 purchase totals = $565.18       $615.0458   $615.04   $615.00   $615.10