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 $20.85   6.557%   $22.2171   $22.22   $22.20   $22.20  
  Purchase #2 $13.38   4.270%   $13.9513   $13.95   $13.95   $14.00  
  Purchase #3 $148.28   10.431%   $163.7471   $163.75   $163.75   $163.70  
  Purchase #4 $157.72   12.670%   $177.7031   $177.70   $177.70   $177.70  
  Purchase #5 $35.57   12.534%   $40.0283   $40.03   $40.05   $40.00  
  Purchase #6 $28.53   12.693%   $32.1513   $32.15   $32.15   $32.20  
  Purchase #7 $170.98   10.642%   $189.1757   $189.18   $189.20   $189.20  
  7 purchase totals = $575.31       $638.9739   $638.98   $639.00   $639.00