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 $19.50   7.622%   $20.9863   $20.99   $21.00   $21.00  
  Purchase #2 $154.83   6.383%   $164.7128   $164.71   $164.70   $164.70  
  Purchase #3 $126.41   10.436%   $139.6021   $139.60   $139.60   $139.60  
  Purchase #4 $152.43   5.198%   $160.3533   $160.35   $160.35   $160.40  
  Purchase #5 $153.54   10.415%   $169.5312   $169.53   $169.55   $169.50  
  Purchase #6 $57.65   11.309%   $64.1696   $64.17   $64.15   $64.20  
  Purchase #7 $70.96   13.434%   $80.4928   $80.49   $80.50   $80.50  
  7 purchase totals = $735.32       $799.8481   $799.84   $799.85   $799.90