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 $53.71   6.122%   $56.9981   $57.00   $57.00   $57.00  
  Purchase #2 $166.95   5.196%   $175.6247   $175.62   $175.60   $175.60  
  Purchase #3 $189.97   6.382%   $202.0939   $202.09   $202.10   $202.10  
  Purchase #4 $90.37   7.160%   $96.8405   $96.84   $96.85   $96.80  
  Purchase #5 $44.58   10.673%   $49.338   $49.34   $49.35   $49.30  
  Purchase #6 $206.69   9.107%   $225.5133   $225.51   $225.50   $225.50  
  Purchase #7 $190.84   3.282%   $197.1034   $197.10   $197.10   $197.10  
  7 purchase totals = $943.11       $1,003.5119   $1,003.50   $1,003.50   $1,003.40