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 $18.31   9.172%   $19.9894   $19.99   $20.00   $20.00  
  Purchase #2 $141.83   11.236%   $157.766   $157.77   $157.75   $157.80  
  Purchase #3 $138.88   9.286%   $151.7764   $151.78   $151.80   $151.80  
  Purchase #4 $57.30   9.555%   $62.775   $62.78   $62.80   $62.80  
  Purchase #5 $7.49   9.658%   $8.2134   $8.21   $8.20   $8.20  
  Purchase #6 $147.36   9.839%   $161.8588   $161.86   $161.85   $161.90  
  Purchase #7 $91.83   4.324%   $95.8007   $95.80   $95.80   $95.80  
  7 purchase totals = $603.00       $658.1797   $658.19   $658.20   $658.30