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 $33.39   13.177%   $37.7898   $37.79   $37.80   $37.80  
  Purchase #2 $61.31   10.277%   $67.6108   $67.61   $67.60   $67.60  
  Purchase #3 $88.73   6.757%   $94.7255   $94.73   $94.75   $94.70  
  Purchase #4 $100.79   12.645%   $113.5349   $113.53   $113.55   $113.50  
  Purchase #5 $228.71   9.856%   $251.2517   $251.25   $251.25   $251.30  
  Purchase #6 $219.56   11.779%   $245.422   $245.42   $245.40   $245.40  
  Purchase #7 $178.64   12.528%   $201.02   $201.02   $201.00   $201.00  
  7 purchase totals = $911.13       $1,011.3547   $1,011.35   $1,011.35   $1,011.30