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 $50.90   12.589%   $57.3078   $57.31   $57.30   $57.30  
  Purchase #2 $228.85   11.254%   $254.6048   $254.60   $254.60   $254.60  
  Purchase #3 $132.36   11.419%   $147.4742   $147.47   $147.45   $147.50  
  Purchase #4 $175.61   4.508%   $183.5265   $183.53   $183.55   $183.50  
  Purchase #5 $27.82   12.282%   $31.2369   $31.24   $31.25   $31.20  
  Purchase #6 $110.87   7.322%   $118.9879   $118.99   $119.00   $119.00  
  Purchase #7 $10.33   4.565%   $10.8016   $10.80   $10.80   $10.80  
  7 purchase totals = $736.74       $803.9397   $803.94   $803.95   $803.90