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 $76.70   9.553%   $84.0272   $84.03   $84.05   $84.00  
  Purchase #2 $224.86   6.303%   $239.0329   $239.03   $239.05   $239.00  
  Purchase #3 $71.84   7.492%   $77.2223   $77.22   $77.20   $77.20  
  Purchase #4 $206.93   7.755%   $222.9774   $222.98   $223.00   $223.00  
  Purchase #5 $52.64   8.314%   $57.0165   $57.02   $57.00   $57.00  
  Purchase #6 $207.54   11.434%   $231.2701   $231.27   $231.25   $231.30  
  Purchase #7 $74.75   13.384%   $84.7545   $84.75   $84.75   $84.80  
  7 purchase totals = $915.26       $996.3009   $996.30   $996.30   $996.30