Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $121.67   7.821%   $131.1858   $131.19   $131.20   $131.20  
  Purchase #2 $199.60   12.757%   $225.063   $225.06   $225.05   $225.10  
  Purchase #3 $136.39   10.283%   $150.415   $150.42   $150.40   $150.40  
  Purchase #4 $95.87   9.364%   $104.8473   $104.85   $104.85   $104.80  
  Purchase #5 $64.52   5.581%   $68.1209   $68.12   $68.10   $68.10  
  Purchase #6 $38.25   5.477%   $40.345   $40.35   $40.35   $40.30  
  Purchase #7 $169.81   13.429%   $192.6138   $192.61   $192.60   $192.60  
  7 purchase totals = $826.11       $912.5908   $912.60   $912.55   $912.50