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 $226.64   10.726%   $250.9494   $250.95   $250.95   $250.90  
  Purchase #2 $178.92   7.145%   $191.7038   $191.70   $191.70   $191.70  
  Purchase #3 $118.73   7.873%   $128.0776   $128.08   $128.10   $128.10  
  Purchase #4 $199.58   13.640%   $226.8027   $226.80   $226.80   $226.80  
  Purchase #5 $135.72   6.858%   $145.0277   $145.03   $145.05   $145.00  
  Purchase #6 $12.33   9.261%   $13.4719   $13.47   $13.45   $13.50  
  Purchase #7 $238.31   5.157%   $250.5996   $250.60   $250.60   $250.60  
  7 purchase totals = $1,110.23       $1,206.6327   $1,206.63   $1,206.65   $1,206.60