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 $56.90   5.712%   $60.1501   $60.15   $60.15   $60.20  
  Purchase #2 $66.53   12.814%   $75.0552   $75.06   $75.05   $75.10  
  Purchase #3 $176.88   11.165%   $196.6287   $196.63   $196.65   $196.60  
  Purchase #4 $31.97   3.855%   $33.2024   $33.20   $33.20   $33.20  
  Purchase #5 $207.31   12.666%   $233.5679   $233.57   $233.55   $233.60  
  Purchase #6 $246.78   6.565%   $262.9811   $262.98   $263.00   $263.00  
  Purchase #7 $57.85   3.363%   $59.7955   $59.80   $59.80   $59.80  
  7 purchase totals = $844.22       $921.3809   $921.39   $921.40   $921.50