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 $249.89   9.753%   $274.2618   $274.26   $274.25   $274.30  
  Purchase #2 $219.69   12.232%   $246.5625   $246.56   $246.55   $246.60  
  Purchase #3 $202.58   11.675%   $226.2312   $226.23   $226.25   $226.20  
  Purchase #4 $187.95   13.748%   $213.7894   $213.79   $213.80   $213.80  
  Purchase #5 $203.49   5.830%   $215.3535   $215.35   $215.35   $215.40  
  Purchase #6 $216.50   6.198%   $229.9187   $229.92   $229.90   $229.90  
  Purchase #7 $233.93   12.576%   $263.349   $263.35   $263.35   $263.30  
  7 purchase totals = $1,514.03       $1,669.4661   $1,669.46   $1,669.45   $1,669.50