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 $45.93   12.157%   $51.5137   $51.51   $51.50   $51.50  
  Purchase #2 $230.96   11.596%   $257.7421   $257.74   $257.75   $257.70  
  Purchase #3 $103.40   13.705%   $117.571   $117.57   $117.55   $117.60  
  Purchase #4 $226.70   12.770%   $255.6496   $255.65   $255.65   $255.60  
  Purchase #5 $54.70   10.841%   $60.63   $60.63   $60.65   $60.60  
  Purchase #6 $22.81   8.461%   $24.74   $24.74   $24.75   $24.70  
  Purchase #7 $22.51   7.854%   $24.2779   $24.28   $24.30   $24.30  
  7 purchase totals = $707.01       $792.1243   $792.12   $792.15   $792.00