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 $244.26   13.254%   $276.6342   $276.63   $276.65   $276.60  
  Purchase #2 $226.38   12.397%   $254.4443   $254.44   $254.45   $254.40  
  Purchase #3 $95.60   8.297%   $103.5319   $103.53   $103.55   $103.50  
  Purchase #4 $9.92   11.343%   $11.0452   $11.05   $11.05   $11.00  
  Purchase #5 $60.45   11.376%   $67.3268   $67.33   $67.35   $67.30  
  Purchase #6 $64.60   3.151%   $66.6355   $66.64   $66.65   $66.60  
  Purchase #7 $204.35   3.549%   $211.6024   $211.60   $211.60   $211.60  
  7 purchase totals = $905.56       $991.2203   $991.22   $991.30   $991.00