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 $16.86   11.155%   $18.7407   $18.74   $18.75   $18.70  
  Purchase #2 $189.58   3.247%   $195.7357   $195.74   $195.75   $195.70  
  Purchase #3 $210.49   4.452%   $219.861   $219.86   $219.85   $219.90  
  Purchase #4 $242.28   11.785%   $270.8327   $270.83   $270.85   $270.80  
  Purchase #5 $11.82   12.566%   $13.3053   $13.31   $13.30   $13.30  
  Purchase #6 $10.29   12.783%   $11.6054   $11.61   $11.60   $11.60  
  Purchase #7 $235.42   11.375%   $262.199   $262.20   $262.20   $262.20  
  7 purchase totals = $916.74       $992.2798   $992.29   $992.30   $992.20