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 $218.66   12.701%   $246.432   $246.43   $246.45   $246.40  
  Purchase #2 $79.85   13.436%   $90.5786   $90.58   $90.60   $90.60  
  Purchase #3 $75.39   6.802%   $80.518   $80.52   $80.50   $80.50  
  Purchase #4 $24.49   11.661%   $27.3458   $27.35   $27.35   $27.30  
  Purchase #5 $216.50   11.220%   $240.7913   $240.79   $240.80   $240.80  
  Purchase #6 $23.47   7.360%   $25.1974   $25.20   $25.20   $25.20  
  Purchase #7 $192.57   12.167%   $216   $216.00   $216.00   $216.00  
  7 purchase totals = $830.93       $926.8631   $926.87   $926.90   $926.80