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 $16.99   10.572%   $18.7862   $18.79   $18.80   $18.80  
  Purchase #2 $166.94   7.822%   $179.998   $180.00   $180.00   $180.00  
  Purchase #3 $29.25   12.155%   $32.8053   $32.81   $32.80   $32.80  
  Purchase #4 $204.75   7.580%   $220.2701   $220.27   $220.25   $220.30  
  Purchase #5 $245.91   8.368%   $266.4877   $266.49   $266.50   $266.50  
  Purchase #6 $182.54   3.104%   $188.206   $188.21   $188.20   $188.20  
  Purchase #7 $223.28   4.566%   $233.475   $233.48   $233.50   $233.50  
  7 purchase totals = $1,069.66       $1,140.0283   $1,140.05   $1,140.05   $1,140.10