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 $74.59   7.155%   $79.9269   $79.93   $79.95   $79.90  
  Purchase #2 $192.35   6.436%   $204.7296   $204.73   $204.75   $204.70  
  Purchase #3 $119.91   4.614%   $125.4426   $125.44   $125.45   $125.40  
  Purchase #4 $191.75   4.431%   $200.2464   $200.25   $200.25   $200.20  
  Purchase #5 $61.34   8.463%   $66.5312   $66.53   $66.55   $66.50  
  Purchase #6 $129.68   9.610%   $142.1422   $142.14   $142.15   $142.10  
  Purchase #7 $159.93   9.806%   $175.6127   $175.61   $175.60   $175.60  
  7 purchase totals = $929.55       $994.6316   $994.63   $994.70   $994.40