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 $18.92   4.619%   $19.7939   $19.79   $19.80   $19.80  
  Purchase #2 $96.81   9.821%   $106.3177   $106.32   $106.30   $106.30  
  Purchase #3 $149.85   11.268%   $166.7351   $166.74   $166.75   $166.70  
  Purchase #4 $105.77   13.173%   $119.7031   $119.70   $119.70   $119.70  
  Purchase #5 $128.85   3.687%   $133.6007   $133.60   $133.60   $133.60  
  Purchase #6 $128.34   7.690%   $138.2093   $138.21   $138.20   $138.20  
  Purchase #7 $145.86   8.767%   $158.6475   $158.65   $158.65   $158.60  
  7 purchase totals = $774.40       $843.0073   $843.01   $843.00   $842.90