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 $6.49   5.448%   $6.8436   $6.84   $6.85   $6.80  
  Purchase #2 $195.38   4.571%   $204.3108   $204.31   $204.30   $204.30  
  Purchase #3 $193.90   12.128%   $217.4162   $217.42   $217.40   $217.40  
  Purchase #4 $122.34   4.740%   $128.1389   $128.14   $128.15   $128.10  
  Purchase #5 $178.79   9.221%   $195.2762   $195.28   $195.30   $195.30  
  Purchase #6 $87.78   7.848%   $94.669   $94.67   $94.65   $94.70  
  Purchase #7 $85.37   5.756%   $90.2839   $90.28   $90.30   $90.30  
  7 purchase totals = $870.05       $936.9386   $936.94   $936.95   $936.90