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 $13.93   13.727%   $15.8422   $15.84   $15.85   $15.80  
  Purchase #2 $147.29   11.356%   $164.0163   $164.02   $164.00   $164.00  
  Purchase #3 $104.95   10.644%   $116.1209   $116.12   $116.10   $116.10  
  Purchase #4 $92.55   12.197%   $103.8383   $103.84   $103.85   $103.80  
  Purchase #5 $74.36   7.655%   $80.0523   $80.05   $80.05   $80.10  
  Purchase #6 $7.70   8.844%   $8.381   $8.38   $8.40   $8.40  
  Purchase #7 $30.28   9.389%   $33.123   $33.12   $33.10   $33.10  
  7 purchase totals = $471.06       $521.3740   $521.37   $521.35   $521.30