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 $249.29   10.407%   $275.2336   $275.23   $275.25   $275.20  
  Purchase #2 $194.37   6.773%   $207.5347   $207.53   $207.55   $207.50  
  Purchase #3 $229.77   11.233%   $255.5801   $255.58   $255.60   $255.60  
  Purchase #4 $231.72   9.411%   $253.5272   $253.53   $253.55   $253.50  
  Purchase #5 $26.42   12.579%   $29.7434   $29.74   $29.75   $29.70  
  Purchase #6 $203.41   7.799%   $219.2739   $219.27   $219.25   $219.30  
  Purchase #7 $7.53   6.198%   $7.9967   $8.00   $8.00   $8.00  
  7 purchase totals = $1,142.51       $1,248.8896   $1,248.88   $1,248.95   $1,248.80