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 $18.57   10.755%   $20.5672   $20.57   $20.55   $20.60  
  Purchase #2 $14.35   11.626%   $16.0183   $16.02   $16.00   $16.00  
  Purchase #3 $155.84   7.625%   $167.7228   $167.72   $167.70   $167.70  
  Purchase #4 $26.96   12.674%   $30.3769   $30.38   $30.40   $30.40  
  Purchase #5 $20.38   9.687%   $22.3542   $22.35   $22.35   $22.40  
  Purchase #6 $18.60   4.471%   $19.4316   $19.43   $19.45   $19.40  
  Purchase #7 $70.85   9.868%   $77.8415   $77.84   $77.85   $77.80  
  7 purchase totals = $325.55       $354.3125   $354.31   $354.30   $354.30