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 $28.64   8.869%   $31.1801   $31.18   $31.20   $31.20  
  Purchase #2 $31.40   8.498%   $34.0684   $34.07   $34.05   $34.10  
  Purchase #3 $108.43   10.747%   $120.083   $120.08   $120.10   $120.10  
  Purchase #4 $168.75   6.273%   $179.3357   $179.34   $179.35   $179.30  
  Purchase #5 $20.67   6.377%   $21.9881   $21.99   $22.00   $22.00  
  Purchase #6 $76.73   7.785%   $82.7034   $82.70   $82.70   $82.70  
  Purchase #7 $90.62   11.260%   $100.8238   $100.82   $100.80   $100.80  
  7 purchase totals = $525.24       $570.1825   $570.18   $570.20   $570.20