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 $120.66   6.421%   $128.4076   $128.41   $128.40   $128.40  
  Purchase #2 $74.72   4.720%   $78.2468   $78.25   $78.25   $78.20  
  Purchase #3 $32.32   13.483%   $36.6777   $36.68   $36.70   $36.70  
  Purchase #4 $143.80   4.398%   $150.1243   $150.12   $150.10   $150.10  
  Purchase #5 $10.28   11.572%   $11.4696   $11.47   $11.45   $11.50  
  Purchase #6 $171.81   10.336%   $189.5683   $189.57   $189.55   $189.60  
  Purchase #7 $76.83   10.118%   $84.6037   $84.60   $84.60   $84.60  
  7 purchase totals = $630.42       $679.0980   $679.10   $679.05   $679.10