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 $90.80   9.642%   $99.5549   $99.55   $99.55   $99.60  
  Purchase #2 $159.58   6.619%   $170.1426   $170.14   $170.15   $170.10  
  Purchase #3 $151.30   5.760%   $160.0149   $160.01   $160.00   $160.00  
  Purchase #4 $6.90   3.712%   $7.1561   $7.16   $7.15   $7.20  
  Purchase #5 $103.95   6.620%   $110.8315   $110.83   $110.85   $110.80  
  Purchase #6 $9.31   3.665%   $9.6512   $9.65   $9.65   $9.70  
  Purchase #7 $41.96   13.602%   $47.6674   $47.67   $47.65   $47.70  
  7 purchase totals = $563.80       $605.0186   $605.01   $605.00   $605.10