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 $19.85   4.163%   $20.6764   $20.68   $20.70   $20.70  
  Purchase #2 $39.63   6.452%   $42.1869   $42.19   $42.20   $42.20  
  Purchase #3 $21.95   12.704%   $24.7385   $24.74   $24.75   $24.70  
  Purchase #4 $138.95   11.449%   $154.8584   $154.86   $154.85   $154.90  
  Purchase #5 $219.59   6.329%   $233.4879   $233.49   $233.50   $233.50  
  Purchase #6 $55.30   9.330%   $60.4595   $60.46   $60.45   $60.50  
  Purchase #7 $135.81   10.192%   $149.6518   $149.65   $149.65   $149.70  
  7 purchase totals = $631.08       $686.0594   $686.07   $686.10   $686.20