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 $30.55   3.654%   $31.6663   $31.67   $31.65   $31.70  
  Purchase #2 $70.98   12.431%   $79.8035   $79.80   $79.80   $79.80  
  Purchase #3 $150.92   8.811%   $164.2176   $164.22   $164.20   $164.20  
  Purchase #4 $112.32   11.856%   $125.6367   $125.64   $125.65   $125.60  
  Purchase #5 $81.79   13.828%   $93.0999   $93.10   $93.10   $93.10  
  Purchase #6 $95.90   4.431%   $100.1493   $100.15   $100.15   $100.10  
  Purchase #7 $39.30   9.203%   $42.9168   $42.92   $42.90   $42.90  
  7 purchase totals = $581.76       $637.4901   $637.50   $637.45   $637.40