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 $247.95   4.366%   $258.7755   $258.78   $258.80   $258.80  
  Purchase #2 $180.85   11.378%   $201.4271   $201.43   $201.45   $201.40  
  Purchase #3 $130.85   5.692%   $138.298   $138.30   $138.30   $138.30  
  Purchase #4 $221.65   6.314%   $235.645   $235.65   $235.65   $235.60  
  Purchase #5 $17.60   11.474%   $19.6194   $19.62   $19.60   $19.60  
  Purchase #6 $62.48   3.757%   $64.8274   $64.83   $64.85   $64.80  
  Purchase #7 $155.65   6.107%   $165.1555   $165.16   $165.15   $165.20  
  7 purchase totals = $1,017.03       $1,083.7479   $1,083.77   $1,083.80   $1,083.70