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 $19.72   5.795%   $20.8628   $20.86   $20.85   $20.90  
  Purchase #2 $89.56   7.439%   $96.2224   $96.22   $96.20   $96.20  
  Purchase #3 $245.55   6.807%   $262.2646   $262.26   $262.25   $262.30  
  Purchase #4 $121.43   7.594%   $130.6514   $130.65   $130.65   $130.70  
  Purchase #5 $131.35   3.419%   $135.8409   $135.84   $135.85   $135.80  
  Purchase #6 $103.70   3.483%   $107.3119   $107.31   $107.30   $107.30  
  Purchase #7 $218.74   8.427%   $237.1732   $237.17   $237.15   $237.20  
  7 purchase totals = $930.05       $990.3272   $990.31   $990.25   $990.40