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 $18.73   4.138%   $19.505   $19.51   $19.50   $19.50  
  Purchase #2 $119.51   7.655%   $128.6585   $128.66   $128.65   $128.70  
  Purchase #3 $215.85   9.395%   $236.1291   $236.13   $236.15   $236.10  
  Purchase #4 $185.57   9.469%   $203.1416   $203.14   $203.15   $203.10  
  Purchase #5 $19.53   7.578%   $21.01   $21.01   $21.00   $21.00  
  Purchase #6 $184.37   11.429%   $205.4416   $205.44   $205.45   $205.40  
  Purchase #7 $233.88   3.703%   $242.5406   $242.54   $242.55   $242.50  
  7 purchase totals = $977.44       $1,056.4264   $1,056.43   $1,056.45   $1,056.30