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 $12.64   9.820%   $13.8812   $13.88   $13.90   $13.90  
  Purchase #2 $172.81   5.375%   $182.0985   $182.10   $182.10   $182.10  
  Purchase #3 $20.68   7.539%   $22.2391   $22.24   $22.25   $22.20  
  Purchase #4 $214.56   4.857%   $224.9812   $224.98   $225.00   $225.00  
  Purchase #5 $189.79   13.707%   $215.8045   $215.80   $215.80   $215.80  
  Purchase #6 $64.59   8.744%   $70.2377   $70.24   $70.25   $70.20  
  Purchase #7 $105.94   7.327%   $113.7022   $113.70   $113.70   $113.70  
  7 purchase totals = $781.01       $842.9444   $842.94   $843.00   $842.90