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 $19.80   4.419%   $20.675   $20.68   $20.70   $20.70  
  Purchase #2 $162.67   5.750%   $172.0235   $172.02   $172.00   $172.00  
  Purchase #3 $217.79   7.392%   $233.889   $233.89   $233.90   $233.90  
  Purchase #4 $128.30   11.472%   $143.0186   $143.02   $143.00   $143.00  
  Purchase #5 $17.62   7.642%   $18.9665   $18.97   $18.95   $19.00  
  Purchase #6 $192.54   4.162%   $200.5535   $200.55   $200.55   $200.60  
  Purchase #7 $189.85   9.485%   $207.8573   $207.86   $207.85   $207.90  
  7 purchase totals = $928.57       $996.9834   $996.99   $996.95   $997.10