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 $244.41   12.754%   $275.5821   $275.58   $275.60   $275.60  
  Purchase #2 $190.56   9.871%   $209.3702   $209.37   $209.35   $209.40  
  Purchase #3 $69.42   9.430%   $75.9663   $75.97   $75.95   $76.00  
  Purchase #4 $120.36   9.308%   $131.5631   $131.56   $131.55   $131.60  
  Purchase #5 $220.56   4.173%   $229.764   $229.76   $229.75   $229.80  
  Purchase #6 $234.80   8.362%   $254.434   $254.43   $254.45   $254.40  
  Purchase #7 $58.35   12.520%   $65.6554   $65.66   $65.65   $65.70  
  7 purchase totals = $1,138.46       $1,242.3351   $1,242.33   $1,242.30   $1,242.50