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 $109.93   10.628%   $121.6134   $121.61   $121.60   $121.60  
  Purchase #2 $242.57   7.691%   $261.2261   $261.23   $261.25   $261.20  
  Purchase #3 $162.45   9.345%   $177.631   $177.63   $177.65   $177.60  
  Purchase #4 $197.78   7.821%   $213.2484   $213.25   $213.25   $213.20  
  Purchase #5 $155.43   10.400%   $171.5947   $171.59   $171.60   $171.60  
  Purchase #6 $120.93   11.787%   $135.184   $135.18   $135.20   $135.20  
  Purchase #7 $197.31   7.503%   $212.1142   $212.11   $212.10   $212.10  
  7 purchase totals = $1,186.40       $1,292.6118   $1,292.60   $1,292.65   $1,292.50