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 $167.69   7.335%   $179.9901   $179.99   $180.00   $180.00  
  Purchase #2 $159.70   9.704%   $175.1973   $175.20   $175.20   $175.20  
  Purchase #3 $182.92   3.858%   $189.9771   $189.98   $190.00   $190.00  
  Purchase #4 $150.54   11.814%   $168.3248   $168.32   $168.30   $168.30  
  Purchase #5 $65.47   9.755%   $71.8566   $71.86   $71.85   $71.90  
  Purchase #6 $165.78   5.751%   $175.314   $175.31   $175.30   $175.30  
  Purchase #7 $116.32   11.302%   $129.4665   $129.47   $129.45   $129.50  
  7 purchase totals = $1,008.42       $1,090.1264   $1,090.13   $1,090.10   $1,090.20