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 $24.98   6.552%   $26.6167   $26.62   $26.60   $26.60  
  Purchase #2 $83.47   9.837%   $91.6809   $91.68   $91.70   $91.70  
  Purchase #3 $171.95   7.174%   $184.2857   $184.29   $184.30   $184.30  
  Purchase #4 $109.58   9.374%   $119.852   $119.85   $119.85   $119.90  
  Purchase #5 $78.73   13.741%   $89.5483   $89.55   $89.55   $89.50  
  Purchase #6 $186.68   12.685%   $210.3604   $210.36   $210.35   $210.40  
  Purchase #7 $73.95   10.123%   $81.436   $81.44   $81.45   $81.40  
  7 purchase totals = $729.34       $803.7800   $803.79   $803.80   $803.80