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 $234.39   12.647%   $264.0333   $264.03   $264.05   $264.00  
  Purchase #2 $33.57   5.613%   $35.4543   $35.45   $35.45   $35.50  
  Purchase #3 $82.60   5.752%   $87.3512   $87.35   $87.35   $87.40  
  Purchase #4 $64.54   13.149%   $73.0264   $73.03   $73.05   $73.00  
  Purchase #5 $99.87   4.144%   $104.0086   $104.01   $104.00   $104.00  
  Purchase #6 $145.97   3.142%   $150.5564   $150.56   $150.55   $150.60  
  Purchase #7 $186.72   10.431%   $206.1968   $206.20   $206.20   $206.20  
  7 purchase totals = $847.66       $920.6270   $920.63   $920.65   $920.70