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 $121.98   10.676%   $135.0026   $135.00   $135.00   $135.00  
  Purchase #2 $243.87   13.267%   $276.2242   $276.22   $276.20   $276.20  
  Purchase #3 $243.68   12.813%   $274.9027   $274.90   $274.90   $274.90  
  Purchase #4 $234.89   10.427%   $259.382   $259.38   $259.40   $259.40  
  Purchase #5 $151.55   13.427%   $171.8986   $171.90   $171.90   $171.90  
  Purchase #6 $81.84   12.280%   $91.89   $91.89   $91.90   $91.90  
  Purchase #7 $115.97   3.140%   $119.6115   $119.61   $119.60   $119.60  
  7 purchase totals = $1,193.78       $1,328.9116   $1,328.90   $1,328.90   $1,328.90