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 $27.88   7.551%   $29.9852   $29.99   $30.00   $30.00  
  Purchase #2 $99.48   4.404%   $103.8611   $103.86   $103.85   $103.90  
  Purchase #3 $193.76   13.855%   $220.6054   $220.61   $220.60   $220.60  
  Purchase #4 $100.51   5.134%   $105.6702   $105.67   $105.65   $105.70  
  Purchase #5 $54.99   5.103%   $57.7961   $57.80   $57.80   $57.80  
  Purchase #6 $171.89   7.565%   $184.8935   $184.89   $184.90   $184.90  
  Purchase #7 $161.60   12.823%   $182.322   $182.32   $182.30   $182.30  
  7 purchase totals = $810.11       $885.1335   $885.14   $885.10   $885.20