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 $246.98   4.193%   $257.3359   $257.34   $257.35   $257.30  
  Purchase #2 $54.29   13.297%   $61.5089   $61.51   $61.50   $61.50  
  Purchase #3 $135.37   7.526%   $145.5579   $145.56   $145.55   $145.60  
  Purchase #4 $228.64   3.738%   $237.1866   $237.19   $237.20   $237.20  
  Purchase #5 $13.34   8.355%   $14.4546   $14.45   $14.45   $14.50  
  Purchase #6 $222.50   3.769%   $230.886   $230.89   $230.90   $230.90  
  Purchase #7 $70.99   5.391%   $74.8171   $74.82   $74.80   $74.80  
  7 purchase totals = $972.11       $1,021.7470   $1,021.76   $1,021.75   $1,021.80