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 $17.72   5.406%   $18.6779   $18.68   $18.70   $18.70  
  Purchase #2 $178.58   12.143%   $200.265   $200.27   $200.25   $200.30  
  Purchase #3 $173.87   8.111%   $187.9726   $187.97   $187.95   $188.00  
  Purchase #4 $220.84   8.331%   $239.2382   $239.24   $239.25   $239.20  
  Purchase #5 $127.48   12.759%   $143.7452   $143.75   $143.75   $143.70  
  Purchase #6 $224.32   4.629%   $234.7038   $234.70   $234.70   $234.70  
  Purchase #7 $42.86   6.127%   $45.486   $45.49   $45.50   $45.50  
  7 purchase totals = $985.67       $1,070.0887   $1,070.10   $1,070.10   $1,070.10