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 $226.62   10.310%   $249.9845   $249.98   $250.00   $250.00  
  Purchase #2 $71.88   4.810%   $75.3374   $75.34   $75.35   $75.30  
  Purchase #3 $143.93   6.179%   $152.8234   $152.82   $152.80   $152.80  
  Purchase #4 $99.86   13.583%   $113.424   $113.42   $113.40   $113.40  
  Purchase #5 $185.94   8.454%   $201.6594   $201.66   $201.65   $201.70  
  Purchase #6 $112.57   12.275%   $126.388   $126.39   $126.40   $126.40  
  Purchase #7 $232.78   10.107%   $256.3071   $256.31   $256.30   $256.30  
  7 purchase totals = $1,073.58       $1,175.9238   $1,175.92   $1,175.90   $1,175.90