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 $19.96   6.334%   $21.2243   $21.22   $21.20   $21.20  
  Purchase #2 $61.88   9.660%   $67.8576   $67.86   $67.85   $67.90  
  Purchase #3 $60.62   8.624%   $65.8479   $65.85   $65.85   $65.80  
  Purchase #4 $25.74   9.327%   $28.1408   $28.14   $28.15   $28.10  
  Purchase #5 $198.64   11.161%   $220.8102   $220.81   $220.80   $220.80  
  Purchase #6 $223.67   10.762%   $247.7414   $247.74   $247.75   $247.70  
  Purchase #7 $28.99   10.109%   $31.9206   $31.92   $31.90   $31.90  
  7 purchase totals = $619.50       $683.5428   $683.54   $683.50   $683.40