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 $228.72   9.766%   $251.0568   $251.06   $251.05   $251.10  
  Purchase #2 $6.33   8.285%   $6.8544   $6.85   $6.85   $6.90  
  Purchase #3 $148.64   11.475%   $165.6964   $165.70   $165.70   $165.70  
  Purchase #4 $31.41   13.551%   $35.6664   $35.67   $35.65   $35.70  
  Purchase #5 $202.27   10.353%   $223.211   $223.21   $223.20   $223.20  
  Purchase #6 $81.46   8.674%   $88.5258   $88.53   $88.55   $88.50  
  Purchase #7 $20.98   11.226%   $23.3352   $23.34   $23.35   $23.30  
  7 purchase totals = $719.81       $794.3460   $794.36   $794.35   $794.40