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 $120.28   6.752%   $128.4013   $128.40   $128.40   $128.40  
  Purchase #2 $114.40   12.711%   $128.9414   $128.94   $128.95   $128.90  
  Purchase #3 $233.35   11.129%   $259.3195   $259.32   $259.30   $259.30  
  Purchase #4 $17.29   11.234%   $19.2324   $19.23   $19.25   $19.20  
  Purchase #5 $197.62   11.822%   $220.9826   $220.98   $221.00   $221.00  
  Purchase #6 $123.94   11.292%   $137.9353   $137.94   $137.95   $137.90  
  Purchase #7 $91.97   3.286%   $94.9921   $94.99   $95.00   $95.00  
  7 purchase totals = $898.85       $989.8046   $989.80   $989.85   $989.70